Symposium 343 - Poster Abstracts

 

Stark broadening data for post AGB and AGB stars - STARK-B Database, a node of Virtual Atomic and Molecular Data Center (VAMDC)

Dimitrijevic, Milan S.

Stark broadening of spectral lines is produced by the perturbation of emitting/absorbing atom or ion by interactions with surrounding charged particles. The corresponding Stark broadening parameters, line width and shift, are needed for different applications in Astrophysics (e.g., for stellar atmospheres modelling, analyzis and synthesis of stellar spectra, radiative transfer calculations, abundance determination...), Physics and for industrial plasmas. In atmospheres of hot, massive post AGB stars this is the principal pressure broadening mechanism. It can be of interest and for AGB stars, for subphotospheric layers investigation and modelling. In the case of analysis and synthesis of spectral lines with large principal quantum number, Stark broadening could be non negligible even in deep layers of atmospheres of AGB stars. If needed, Stark broadening parameters can be found in STARK-B database (http://stark-b.obspm.fr), a collaborative project between the "Laboratoire d'Etude du Rayonnement et de la matière en Astrophysique'' (LERMA) of the Paris Observatory and CNRS, and the Astronomical Observatory of Belgrade. We will describe STARK-B as database is on 20. August 2018 . It contains widths and shifts of isolated lines of atoms and ions due to electron and ion impacts (Stark broadening parameters) determined theoretically in more than 150 papers by Dimitrijević, Sahal-Bréchot, and colleagues. We note that STARK-B is one of 33 databases with different atomic and molecular data needed for stellar plasma investigation and modelling within Virtual Atomic and Molecular Data Center (VAMDC – www.vamdc.eu), which will be also described shortly.


Tomography of the red supergiant star mu Cep

Kravchenko, Kateryna

We present a tomographic method allowing to recover the velocity field at different optical depths in a stellar atmosphere. It is based on the computation of the contribution function to identify the depth of formation of spectral lines in order to construct numerical masks probing different optical depths. These masks are cross-correlated with observed spectra to extract information about the average shape of lines forming at a given optical depth and to derive the velocity field projected on the line of sight. We applied this method to series of spectra of the red supergiant star μ Cep and derived velocities in different atmospheric layers. The resulting velocity variations reveal complex atmospheric dynamics and indicate that convective cells are present in the atmosphere of the μ Cep. The μ Cep velocities were compared with those obtained by applying the tomographic masks to series of snapshot spectra from 3D radiative-hydrodynamics CO5BOLD simulations.


A common-envelope wind model for SNe Ia

Meng, Xiangcun

Here, we propose a new version of the SD model in which a common envelope (CE) is assumed_x000D_ to form when the mass-transfer rate between a carbon–oxygen white dwarf (CO WD) and_x000D_ its companion exceeds a critical accretion rate. In our model, helium flash and hydrogen-burning phases adjust themselves just as they do in a TPAGB star. In the CEW model, the SN Ia may explode in CE, supersoft X-ray source and recurrent nova phases. The CEW model shares many of the merits of the previous model while avoiding some of its shortcomings. Our model suggeste that SN 2002cx-like and SN Ia-CSM objects would share the same origin.


New limit on delta alpha/alpha from an analysis QSO spectra

Thong, Le

A new stringent limit relating to the variation of the fine-structure constant (alpha) has been extracted from Ritz wavelengths of 27 quasi stellar object (QSO) absorption spectra lines of Fe II. The calculation was combined with laboratory wavelengths and QSO spectra to obtain the result delta alpha/alpha = (0.027+-0.832)x 10 -6 . This result proposes how dedicated astrophysical estimations can improve these limitations in the future and can also constrain space-time variations.


Impact of AGB stars on their circumstellar environments

Mohamed, Shazrene

Recent high-resolution, high-sensitivity observations, e.g., with Herschel, ALMA, and SPHERE, have given us dramatic new views of cool, evolved giants and their circumstellar environments; revealing dusty arcs, bubbles, dense shells, spirals, and equatorial and bipolar outflows. In this talk, we present 3D simulations of the interaction of the outflows of cool giants with nearby (sub-) stellar companions and the interstellar medium. We discuss the physical processes that govern the origin and evolution of the observed complex structures, and the implications for the wide range of related systems, e.g., symbiotic binaries, detached shell sources, runaway stars and (proto-)planetary nebulae.


Differentially rotating white dwarfs with 2DStars

Halabi, Ghina

The effect of stellar rotation is particularly interesting in accreting white dwarfs (WDs). Accreted material from a companion can transfer substantial angular momentum, thus spinning-up the WD [1]. Over five decades investigations [2,3] have shown that rapid rotation has a large influence on the stability of zero temperature WDs. Using 2DStars [4], a two-dimensional stellar evolution code, we investigate the evolution of the rotation profile on the nuclear time-scale of a rapidly differentially rotating cold WD [5,6]. The aim is to determine whether super-MChandrasekhar WDs can be brought into solid-body rotation due to the dissipative effects induced by the electron and turbulent viscosity, and explode as SNe Ia. We employ a fully relativistic equation of state for cold degenerate matter and start with an internal rotation profile that ensures sufficient differential rotation to avoid immediate collapse. _x000D_ _x000D_ References_x000D_ [1] Durisen R. H., 1977, ApJ, 213, 145_x000D_ [2] James R. A., 1964, ApJ, 140, 552_x000D_ [3] Ostriker J. P., Bodenheimer P., 1968, ApJ, 151, 1089 _x000D_ [4] Halabi, Izzard, Tout, Jermyn & Cannon, 2017, MmSAI, 88, 319H_x000D_ [5] Yoon S.-C., Langer N., 2004, A&A, 419, 623_x000D_ [6] Ghosh P., Wheeler J. C., 2017, ApJ, 834, 93


SWAG: Distribution and Kinematics of an Obscured AGB Population toward the Galactic Center

Ott, Juergen

Outflows from AGB stars enrich the Galactic environment with metals and inject mechanical energy into the ISM. Radio spectroscopy can recover both properties through observations of molecular lines. We present results from SWAG: "Survey of Water and Ammonia in the Galactic Center". The survey covers the entire Central Molecular Zone (CMZ), the inner 3.35x0.9deg (~480x130pc) of the Milky Way that contains ~5x10^7 Mo of molecular gas. Although our survey primarily targets the CMZ, we observe across the entire sightline through the Milky Way, covering the full radial profile of the Galaxy's AGB population. AGB stars are revealed by their signature of double peaked 22GHz water maser lines. They are distinguished by their spectral signatures and their luminosities, which reach up to 10^-7 Lo. Higher luminosities are usually associated with Young Stellar Objects located in CMZ star forming regions. We detect a population of ~600 new water masers that can be associated with AGB outflows. That corresponds to a density of ~200/deg^2, with a Galactic Latitude and Longitude density gradient. Our survey allows us to study the distribution and energetics of the AGB population as well as correlations with additional ~42 thermal molecular lines that we observe in SWAG.


Circumstellar Molecular Maser Emission of AGB and Post-AGB Stars

Rudnitskij, Georgij

Results of long-term studies of circumstellar molecular maser emission of late-type giant and supergiant stars are reported. The observations have been carried out during several decades in the H2O line at a wavelength of 1.35 cm (22-meter telescope in Pushchino, Russia) and in the hydroxyl (OH) lines at 18 cm (Nancay radio telescope, France). A sample of ~70 AGB long-period variable stars has been monitored in the 1.35-cm H2O line in 1980-2018. It includes Mira-type stars (U Ori, RS Vir, U Her, R Cas,…) and semiregular variables (R Crt, RT Vir, W Hya, VX Sgr,…) We have traced H2O maser variations and found them to follow the optical brightness variations with a time delay of 0.3-0.4 stellar period. Some strong H2O flares were observed, which occur every 10 to 15 periods, probably due to long-term changes in the mass-loss rate; good examples are U Ori, U Her, and RS Vir. Especially spectacular flares were demonstrated by the semiregular star W Hya, when its H2O peak flux density reached several thousand janskys, while on the average it did not exceed 50-100 Jy. The model of shock excitation of the H2O maser in combination with stellar radiation pumping is discussed. The same sample of AGB stars was observed in the 18-cm OH lines. For 53 of them, the emission was detected in at least one of three OH lines (1612, 1665, or 1667 MHz). Circular and linear polarization of the maser emission was measured, yielding all four Stokes parameters. Features probably due to Zeeman splitting were detected in the OH line profiles of several stars. Estimated magnetic-field strengths in the maser sources are a few milligauss. In particular, we discuss the data of 2007-2018 on the OH emission of the unique symbiotic star V627 Cas (AS 501, AFGL 2999) displaying maser emission in three ground-sate OH lines 1612, 1665, and 1667 MHz, with remarkable redistribution with time of their relative intensities, as well the new data on the OH and H2O emission of the post-AGB star AI CMi.


GK Car and GZ Nor: Two depleted low-luminosity Galactic RV Tauri stars

Gezer, Ilknur

We present a chemical abundance analyses of two RV Tauri stars GK Car and GZ Nor, on the basis of high-resolution optical spectra obtained with the UVES spectrograph mounted on the 8m UT2 Kueyen Telescope of the VLT array at the Paranal Observatory of ESO in Chili. With an [Fe/H]=-1.32 and a [Zn/Ti]=+1.24 for GK Car and a [Fe/H]=-2.08 and a [Zn/Ti]=+0.77 for GZ Nor, both stars show depletion of refractory elements in their photospheres. Using ASAS photometry, the accurate pulsation periods have been obtained via period analysis. The pulsations of the stars have been used to obtain their luminosities and distances via the PLC relation (Manick et al. 2016). The distance of GK Car is 4.9+/-0.4 kpc and its luminosity is 1800+/-250 Lsun. While for GZ Nor the distance is 10.2+/-0.9 kpc and the luminosity is 2100+/-300 Lsun. RV Tauri stars classified as post-AGB objects by Jura (1986) to show large IR-excess due to thermal emission from circumstellar matter. The luminosity of a post-AGB object is typically expected between 3000 and 10000 Lsun (Van Wickel, 2003). Also, Bertolami (2016) has recently published a detailed grid of post-AGB sequences computed with updated micro and macrophysics. His study shows that new post-AGB models are even 0.1 to 0.3 dex more luminous than the models of similar mass in the older grids (Vassiliadis & Wood 1994; Blöcker 1995a). Recent surveys in the SMC and LMC, a new class of dusty objects termed as "post-RGB" objects have been discovered by Kamath et al (2015b). These objects are evolved, dusty objects and their stellar parameters are very similar to post-AGB objects, except that they have lower luminosities (100-2500 Lsun) than a post-AGB star. These stars very likely evolve off the RGB because of a strong binary interaction process already occurring on the RGB. In this context, GK Car and GZ Nor might be classified as post-RGB stars. This also would imply that they both are binaries which interacted on the RGB (or AGB).


Comprehensive analyses of panchromatic data sets and photo-ionization modeling of PNe

Ueta, Toshiya

Planetary nebulae (PNe) have been favorite spectroscopic targets because their bright spectra are rich in emission lines. Recent opportunities in the far-IR/sub-mm have yielded spatially-resolved fine-structure line emission maps as well as thermal continuum emission maps for a sample of PNe, filling the remaining major spectral gap in this wavelength range of the multi-wavelength complement of the observational PN data. Exploiting panchromatic PN data from X-ray/UV to sub-mm/radio, we can now perform detailed quantitative investigations into the physical conditions of each of the ionized, atomic, and molecular gas and dust components in PNe with a minimum amount of assumptions. For example, we can perform an entirely empirical accounting of the circumstellar mass and abundances for each of the ionized, atomic, and molecular gas and dust components. We can then confront with the present theoretical understanding of evolutionary models and their roles in the chemical evolution of the Milky Way Galaxy and beyond. We will review our most recent attempts to comprehensively analyze the existing panchromatic data and generate one of the most comprehensive Galactic PN models ever constructed.


Planetary Nebulae detected in the AKARI Far-IR All-Sky Survey Maps

Ueta, Toshiya

The AKARI Infrared Astronomical Satellite produced the far-IR all-sky survey (AFASS) maps at roughly arc-minute spatial resolution, enabling us to investigate the whole sky in the far-IR for objects having surface brightnesses greater than a few to a couple of dozen MJy sr−1. Here, we verify that point-source photometry using the aperture correction method based on the empirical point-spread-function templates derived directly from the AFASS maps reproduces fluxes in the AKARI bright source catalogue (BSC) without any additional correction. This means that far-IR photometry of any source can be done by summing all the pixel values within an appropriately defined aperture of the intended targets in the AFASS maps. We then carry out aperture photometry for Galactic planetary nebula listed in the University of Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory Hα Planetary Nebula (HASH PN) database, establishing far-IR fluxes for Galactic PNe including a few hundreds of additional PNe not listed in the BSC. A comparison between presently derived and BSC catalogue fluxes of these PNe suggest that direct aperture photometry with the AFASS maps should be done to obtain more accurate fluxes for sources that are not necessarily point sources such as PNe.


Herschel Planetary Nebula Survey Plus (HerPlaNS+)

Ueta, Toshiya

HerPlaNS+ (Herschel Planetary Nebula Survey Plus) is a far-IR imaging/spectroscopic survey and archival search of planetary nebulae (PNe) using the Herschel Space Observatory and Herschel Science Archive. In this presentation, we review our simultaneous investigations into both of the gas and dust components of a sample of Galactic PNe to comprehensively understand the physical properties of the nebulae. We find that there is a significant amount of flux in the far-IR emitted by PNe, which allows us to probe the photo-dissociated region of the target PNe, amounting to nearly 80% of the observed circumstellar mass. We will also exploit the high spatial resolution of the data to address the spatial distributions of each of the gas and dust components and the gas-to-dust mass ratio distributions.


Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES): AKARI Mission Programme

Ueta, Toshiya

A far-IR imaging survey of the circumstellar dust shells of 144 evolved stars (mostly AGB stars) was performed as a mission program of the AKARI infrared astronomical satellite using its Far-Infrared Surveyor (FIS) instrument. This survey produced maps of roughly 10 arcmin by 40 arcmin or 10 arcmin by 20 arcmin areas of the sky around the target evolved stars in the four FIS bands at 65, 90, 140, and 160 microns. The detected extended dust shells, most of which show a departure from circular symmetry or offsets with respect to the central star, are found in dozen oxygen-rich and dozen carbon-rich objects. We report far-IR flux measurements made for these objects with the latest surface brightness correction method. Flux measurements for the central star and circumstellar shell are made separated for the first time to allow proper accounting of the amount of the circumstellar cold dust mass.


Morpho-Kinematics of the Circumstellar Environments around Post-AGB Stars

Ueta, Toshiya

We report our investigations into the morpho-kinematics of the circumstellar environs around post-asymptotic giant branch (post-AGB) stars representative of each of the two morphological classes, elliptical and bipolar, by means of ALMA observations using the 12CO and 13CO J=3-2 lines in relation to the distribution of neighboring continuum emission. The issue to be addressed is whether the central torus that seems to be generated by the end of the AGB phase is the consequence of intrinsic bipolar outflows carving out a cavity that defines the bipolar axis or the origin of bipolar outflows steered into the intrinsic biconical openings. We look into how the CO outflow velocity field differs in these nebulae, especially the latitudinal dependence of CO outflows. We also aim at resolving the longitudinal/azimuthal motion to see if the central torus exhibit the Keplerian rotation. The 12CO/13CO distribution is also used as a proxy of the 12C/13C abundance ratio distribution, which can vary due to the change in the 13CO abundance. Based on these pieces of dynamical evidence we will determine the role of the central torus in the CSE shaping during the AGB mass loss.


Unravelling the sulphur chemistry of AGB stars

Danilovich, Taïssa

Sulphur is the tenth most abundant element in the universe and its behaviour in terms of what molecules it forms has been seen to vary for different types of AGB stars. There are clear differences across chemical types, with CS forming more readily in the circumstellar envelopes of carbon stars, while SO and SO2 have only been detected in oxygen-rich stars. However, we have also discovered differences in sulphur chemistry based on the density of the circumstellar envelope (as traced by mass-loss rate divided by expansion velocity). For example, the radial distribution of SO is drastically different between AGB stars with lower and higher density circumstellar envelopes. H2S can be found in high abundances towards higher density oxygen-rich stars, whereas SiS accounts for a significant portion of the circumstellar sulphur for higher density carbon stars. We will discuss these differences across AGB stars and what this implies for the chemical evolution of the circumstellar envelope. Studies of post-AGB stars show us that sulphur is not significantly depleted onto dust during the AGB phase, making it a good tracer of changing circumstellar chemistry across the AGB.


When binaries keep track of recent AGB nucleosynthesis: The Zr - Nb pair in extrinsic stars as an efficient s-process diagnostic

Karinkuzhi, Drisya

Extrinsic stars like Barium, CH and CEMP-s (carbon-enriched metal poor) stars are s-process enriched giants; they owe their chemical peculiarities to a past mass transfer, during which they were polluted by their binary companion, which was at the time an asymptotic giant branch (AGB) star, but now an extinct white dwarf. Hence extrinsic stars are ideal targets to understand and constrain the s-process in low- to intermediate-mass AGB stars. Actually, since the $^{93}Zr/Zr$ isotopic ratio is a sensitive function of the s-process operation temperature (independently of stellar evolution models), and since, in extrinsic stars, $^{93}Zr$ has fully decayed into mono-isotopic$^{93}Nb$, we can use the Nb/Zr abundance ratio to constrain the s-process operation temperature. Adopting the same methodology as in Neyskens et al. (Nature 517, 174–176, 2015), we analyze a sample of highly-enriched extrinsic stars observed with the high-resolution HERMES spectrograph mounted on the MERCATOR telescope (La Palma). We derived the abundances of a large number of heavy elements. They provide more accurate constraints on the s-process operation temperature and therefore on the s-process neutron source. The results are then compared with stellar evolution and nucleosynthesis models. Four stars in our sample show peculiar abundance patterns with high nitrogen abundance and high [Nb/Zr] ratio. We compare the nucleosynthetic profile of the present sample stars with those of CEMP-s, CEMP-rs and CEMP-r stars. One barium star of our sample is potentially identified as the highest-metallicity CEMP-rs stars yet discovered. Whether the i-process could account for its abundance pattern is discussed (Karinkuzhi et al., A&A, submitted).


Spectroscopic binaries among AGB stars

Jorissen, Alain

Very few spectroscopic binaries hosting an AGB component are known so far. We will present a few long-period binaries discovered with the HERMES spectrograph mounted on the Mercator telescope at the Roque de los Muchachos Observatory (La Palma). These discoveries open interesting prospects for diagnosing the interaction of the AGB wind with the companion. A few specific cases will be discussed.


Central Stars of Planetary Nebulae in Galactic Open Clusters: Providing additional data for the White Dwarf Initial-to-Final-Mass Relation

Fragkou, Vasiliki

The accurate (<10%) distances of Galactic star clusters, as defined from color-magnitude diagrams, allow precise determination of the physical parameters of any physically associated Planetary Nebula (PN) and also that of its central star (CSPN). Theoretical cluster isochrones can be used for the determination of the PN progenitor's mass and age, while photometry of its CSPN strongly constrains its intrinsic luminosity and mass given the known distance. The mass of the progenitor star can be related to the PN's chemical characteristics, determined from spectroscopic measurements and furthermore, provides additional data for the widely used white dwarf (WD) initial-to-final mass relation (IFMR). The IFMR is crucial for tracing the development of both carbon and nitrogen in entire galaxies. To date there is only one PN (PHR1315- 6555) confirmed to be physically associated with a Galactic Open Cluster (ESO 96 -SC04). This cluster has a turn-off mass   ? 2.2 M?. Our deep HST photometry was used for the search of the CSPN of this unique PN and in this work, we present our results.


The submm properties of dust in the detached shells around carbon AGB stars

Maercker, Matthias

The origin and properties of dust in the universe, and the contribution from AGB stars, is a fundamental question in galaxy evolution. We constrain the properties of the dust grains in the thin detached shells around the carbon AGB stars R Scl, U Ant, V644 Sco, and DR Ser. The shells were created during recent thermal pulses, and the dust properties play a crucial role in understanding the wind-driving mechanism, the evolution of the star throughout the thermal pulse cycle, and the type and amount of dust returned to the ISM from AGB stars. We use new observations from LABOCA and ALMA to model the entire SED including submm wavelengths. For all objects, we find an excess emission in the submm. Spatially resolved observations confine this excess to the detached shells. However, a straightforward explanation for this excess is still lacking. While very large, cold grains can explain the submm flux, they do not reproduce the overall shape of the SED in the FIR and submm. Other obvious grain properties (e.g., composition or geometry) also do not reproduce the observed SEDs. The results imply that the submm observations probe properties of the dust grains that are not typically considered, but may be critical for a complete understanding of dust around evolved stars. A similar SED shape and submm excess has been seen in observations of the small and large magellanic clouds, and has been attributed to unknown dust properties. If the origin of this excess is the same as for the detached shell sources, this would have important implications on the contribution to the total dust budget from AGB stars to galaxies.


The Role of Shocks in the Detemination of Empirical Abundances for Type-I Planetary Nebulae

Costa, Roberto

Despite the growing use of photoionization models to derive chemical abundances of Planetary Nebulae (PNe), empirical abundances derived with the help of Ionization Correction Factors (ICFs) are still very useful for large samples where frequently no information on the central star is available for most nebulae. This work aims to investigate, in the light of new diagnostic diagrams, the role of shocks in the excitation/ionization of type-I PNe, and their relation with the empirical derivation of chemical abundances. We applied the diagnostics to NGC 2440 and NGC6302, both very well known, the first is multipolar with at least two bipolar components, and the second is bipolar.  Our results indicate that shocks play a crucial role in both nebulae.The presence of shocks reinforces the flux of low ionization lines, and the results show that, for these nebulae, shocks are present indeed. This artificial reinforcement of low ionization lines can lead to wrong chemical abundances, when derived through ICFs, at least for type-I PNe. A comprehensive study of this effect, applied to a larger PNe sample and including all types, ideally a statistically complete sample, would be required to clarify this proposition.


Late Thermal Pulse Evolution Models and the Rapid Evolution of V839 Ara

Lawlor, Timothy

We present evolution calculations from the Asymptotic Giant Branch (AGB) to the Planetary Nebula (PNe) phase for models of mass 1 M? over a range of metallicities from primordial, Z = 10-14, through Z = 0.03. The understanding of these objects plays an important role in galactic evolution and composition. Here, we  particularly focus on Late Thermal Pulse (LTP) models, which are models that experience an intense helium pulse that occurs just following the AGB and causes a rapid looping evolution between the AGB and PN phases.  This transient phase only lasts for decades and centuries while increasing and decreasing in temperature, luminosity and size over many orders of magnitude.  For this and similar phases, we can and have observed stellar evolution in real time (e.g.; Sakurai's Object and FG Sge).  In this paper, we use our LTP models to make comparisons to the central star of the Stingray Nebula, (V839 Ara; SAO 244567). This object has been observed to be rapidly evolving and heating over more than 50 years and is the central star of the youngest known planetary nebula.  These characteristics are expected and seen in LTP models.  However, the observations have proven difficult to reporduce because this central star has a high log(g) but lower temperature compared to models. 


Chemical enrichment of galaxies as the result of organic synthesis in evolved stars

Kwok, Sun

Infrared spectroscopic observations have shown that complex organics with mixed aromatic-aliphatic structures are synthesized in large quantities during the late stages of stellar evolution.  These organics are ejected into the interstellar medium and spread across the galaxy.  Due to the sturdy structures of these organic particles, they can survive through long journeys across the galaxy under strong UV background and shock conditions.  The implications that stellar organics were embedded in the primordial solar nebula will be discussed.


The detailed chemical abundance pattern of the metal-rich Galactic globular cluster NGC 6366

Alves-Brito, Alan

Globular cluster (GC) stars have been known to display chemical inhomogeneities (multiple populations) in their light elements (Li, CNO, Na, Al, Mg and F). While this chemical pattern is likely primordial, there is no consensus about the nature of the polluters, with intermediate-mass Asymptotic Giant Branch (AGB) stars, fast rotating massive stars, massive interacting binaries and supermassive stars being some of the candidates. In this work, we present chemical abundances for a sample of eight red giants in the metal-rich GC NGC 6366, which has been little studied in the literature. We have used high quality data (R > 40,000; S/N > 60) obtained at the VLT (8.2 m) and CFHT (3.6 m) telescopes. The abundances for light and heavy elements were obtained through equivalent widths and spectrum synthesis of individual atomic lines, using the MOOG stellar line analysis program, together with 1D Kurucz’s stellar atmosphere models. The derived abundances were compared with those obtained for stars in different environments (field and GCs). Our outcomes suggest that NGC 6366 is a monometallic GC ([Fe/H] = -0.60 ± 0.03), presenting a very short extension in its Na-O anticorrelation. After taking literature’s results into account, the Na-O diagram shows a barely detectable, if any, Na-O anticorrelation. Also, there is no evidence of Mg-Al anticorrelation and no star-to-star variation in Al in our sample. Abundance ratios of heavier elements, as Ba and Eu, as well as the similarities between the [Eu/Fe] and [a/Fe] ratios, lead us to believe that the primordial composition of NGC 6366 is dominated by objects with short time-scale, likely Type II SNe, with mild contribution of AGBs. None of the elements measured present star-to-star variation greater than the uncertainties, which means that NGC 6366 is chemically homogeneous.


On the circumstellar effects on the Li and Ca abundances in massive Galactic AGB stars

Pérez-Mesa, Víctor

By considering the presence of a circumstellar envelope, we previously found strong circumstellar effects in the determination of the Rb abundances in massive AGB stars; these circumstellar effects, however, were found to be very weak for Zr. The abundances of lighter elements such as Li (and Ca) in massive AGB stars may be altered by the activation of hot bottom burning (HBB), which is expected to overproduce the 7Li (and 41Ca) isotope; e.g., the (super-)Li-rich character of massive AGB stars has been used as a HBB indicator in these stars. Here we explore the circumstellar effects on the Li and Ca abundances determination in massive AGB stars. By using our pseudo-dynamical models, we report new Li abundances in a complete sample of massive Galactic AGB stars, while their Ca abundances are reported for the first time. The circumstellar effects on the Li and Ca abundances are found to be very weak and the Li and Ca abundances derived with the pseudo-dynamical models are similar to those derived with the hydrostatical ones. The Li abundances thus confirm the (super-)Li-rich character of our sample stars and the HBB activation in massive Galactic AGB stars. Remarkably, massive Galactic AGB stars are found to be strongly underabundant in Ca (sometimes by up to 1-2 dex, with respect to the expected solar value). Possible explanations for this unexpected Ca underabundance in massive Galactic AGB stars are offered.


Ammonia in carbon rich stars

Etmanski, Bartosz

Ammonia has been found to exists in circumstellar envelopes of evolved starsin amounts that exceed predictons of the current chemical models for C-rich AGB stars.HIFISTARS Team clearly detected the ground-state rotational transition NH3 JK = 10 -- 00at 572.5 GHz in envelopes of few C-rich AGB stars, including earlierdetection in IRC+10216. We present results of the non-LTE radiativetransfer modelling conducted with the goal to derive NH3 abundances in these sources. The model of ammonia includes IR radiative pumping via ?=1 vibrationalband at 10 µm. Theoretical predictions of the model are applied forthe analysis of excited rotational ammonia lines available for some of thesources, in particular for IRC+10216.


The discovery of an asymmetric detached shell around the "fresh" carbon AGB star TX Psc

Brunner, Magdalena

We present ALMA observations of the circumstellar envelope around the carbon AGB star TX Psc in molecular CO(2-1) emission, where we detect a previously not classified detached shell with filamentary structure and elliptical shape. Detached shells are in theory created through intense mass-loss episodes triggered by thermal pulses, where the mass-loss rate and wind velocity increases for a short period of time (typically a few 100 years). Up to now, all observed detached shells are found around carbon AGB stars and are of remarkable spherical symmetry. The elliptical shell around TX Psc is the first clear exception to that rule, with TX Psc being classified as rather "fresh" carbon star, that most likely has only experienced very few thermal pulses yet.The ALMA observations of molecular gas are consistent with Herschel/PACS observations of thermal dust emission, and the elliptical shape of the shell is almost certainly not caused by interaction with the surrounding interstellar medium, since a clear interaction front is visible in the Herschel/PACS images at slightly larger spatial scales, and the morphology of the present-day wind affirms the intrinsic ellipticity of the shell.We investigate and discuss the 3D structure of the CSE and its most likely formation scenarios, as well as the link of this peculiar detached shell to the AGB evolutionary status of TX Psc.


Heavy puzzle pieces: Learning about the i process from Pb abundances

Hampel, Melanie

The large majority of elements heavier than iron are formed by the slow (s) and rapid (r) neutron capture processes. However, it has become clear that a neutron capture process operating at neutron densities intermediate to the s and r process (i process) gives rise to its own characteristic abundance pattern. This i-process pattern is successful at reproducing observed heavy-element abundances that could not be explained previously, e.g. those of carbon-enhanced metal-poor stars that show enrichments of s- and r-process elements (CEMP-s/r). The required high neutron densities may occur in the thermal pulses of Asymptotic Giant Branch (AGB) stars as a result of proton ingestion episodes. However, the sites of the i process are as yet unknown.Comparing theoretical predictions of i-process nucleosynthesis with the observed abundance patterns of CEMP stars and post-AGB stars in the Magellanic Clouds allows us to learn about the thermodynamic properties of possible i-process sites. In particular the Pb abundances may hold the key to solving this mystery because this is one element that is predicted to be significantly enhanced by the s process at low metallicities, in contrast to observations of post-AGB stars which only show low to moderate Pb enhancements. In this talk I will present the results of nuclear-network calculations of i-process nucleosynthesis in comparison to observations. 


Separation of gas and dust in the winds of AGB stars

Mattsson, Lars

Due to hydrodynamic drag, large dust grains will decouple from a gas flow, while small grains will tend to follow the motions of the gas; the drag is inversely proportional to the Stokes number. Similarly, the drag force radiatively accelerated dust grains exert on the gas is also dependent on the Stokes number. In a stellar-wind context this is often called “drift”, which is referring to the fact that dust and gas may develop different mean-flow velocities. In a spherically-symmetric setting, a mean-flow model of drift is fairly simple. But there are significant 3D effects, which we must take into account. For instance, dust grains of various sizes will cluster and increase the rate of grain-grain interaction. Moreover, due to spatial separation of dust and gas, condensation is likely less effective than in a velocity-coupled case. There are several implications for dust-driven stellar winds which arise from this. One is that the momentum-transfer efficiency (from dust to gas) of large grains may be low as they cluster where the gas is not; another is that the sublimation rate increases as grains are more exposed to the radiation field._x000D_ Here, we present results from high-resolution box simulations of forced turbulence and apply these results on the problem of dust-driven winds from AGB stars. The simulations represent a first set of idealised models, which aim at understanding the classical particle-in-flow problem for hypersonic stochastic gas dynamics, i.e., type of flows we typically encounter in astrophysics. It is argued that modelling of the dust-driven winds of AGB stars is a problem that may need to be treated in a less holistic way, where some parts of the problem is treated separately in detailed simulations and parameterised back into a less detailed model describing the big picture.


Gaseous metal oxides and hydroxides tracing the formation of AGB dust grains.

Decin, Leen

The gas-phase elements Ca, Fe, Mg, Si and Ti are depleted w.r.t. the solar abundances in diffuse clouds. The formation of metal oxides and metal hydroxides and of dust species is suggested as major cause for this depletion. We have used the ALMA telescope to perform a high-sensitivity scan between 335 and 362 GHz of the stellar wind of two oxygen-rich Asymptotic Giant Branch (AGB) stars, IK Tau and R Dor. A particular aim of the observing programme was to detect metal oxides and metal hydroxides (AlO, AlOH, FeO, MgO, MgOH), some of which are thought to be direct precursors of dust nucleation and growth. During this presentation, we focus on two aspects. We report on the potential first detection of FeO (v=0, Omega=4, J=11-10) in the wind of an AGB star (being R Dor) and discuss its formation and depletion mechanisms. Secondly, we show that for both stars, only ~2% of aluminium is locked up in AlO, AlOH, and AlCl, and that each of these species is detected well beyond the main dust condensation region. This proves that the aluminium dust condensation is not 100% efficient. We will discuss how the current proposed scenario of aluminium dust condensation poses a challenge if one wishes to explain both the dust spectral features in the spectral energy distribution(SED), in interferometric data, and in polarized light signal. We postulate that large gas-phase (Al2O3)n-clusters (n>34) can be the potential agents of the broad 11 micron feature in the SED and interferometric data and we explain how these large clusters can be formed.


Determining the effect of a non-uniform AGB outflow on its chemistry

Van de Sande, Marie

The chemistry within the outflow of an AGB star is determined by its elemental C/O abundance ratio. Due to the strong molecular bond of CO, no C-rich species are expected in the inner wind of O-rich stars, as all the carbon is expected to be locked up in CO. Similarly for C-rich stars, where no O-rich species are expected in the inner wind. Inner wind chemical models that take into account the effect of shocks, induced by the pulsating AGB star, can account for the presence of most unexpected species, but not all._x000D_ An alternative mechanism to the formation of the unexpected species is the penetration of harsh interstellar UV photons in a non-uniform outflow. Thanks to the advent of high angular resolution observations, it is clear that most outflows do not have a smooth density distribution, but are distinctly inhomogeneous or "clumpy". In such clumpy outflows, UV photons are able to reach the otherwise shielded inner wind region through the porous channels between the clumps. This leads to the photodissociation of CO, releasing the deficient element and allowing for the formation of the unexpected species in the inner wind._x000D_ We have included a clumpy density distribution in our chemical kinetic model by using a porosity formalism. The formalism provides us with a mathematical framework in which the increased leakage of UV photons is accounted for by modifying the optical depth of the outflow. Moreover, it also allows us to include the effects of the relative overdensity of the clumps. Both the leakage of UV photons and the overdensity of the clumps play an important role in the chemistry throughout the AGB wind. We explore the parameter space characterising the specific clumpiness of the outflow and describe its effect on the chemistry. We find that our results can explain the existence of certain species in the inner wind and add to the results of the non-thermodynamic equilibrium inner wind chemical models.


Rotating disks around M-type AGB stars

homan, ward

High-resolution observations of the circumstellar environments (CSE) of AGB stars reveals that companion-induced morphologies are common. High mass-loss rate C-rich winds occasionally exhibit convincing spirals, whereas O-rich environments seem to be less morphologically well-defined. We present a study on the morphology of the CSE of the M-type AGB star EP Aquarii, observed with ALMA (cycle 4). The data reveals a complex, nearly face-on equatorial density enhancement (EDE), enclosed by a bi-conical outflow. The EDE manifests primarily as a spiral, making this the first convincing detection of a spiral in an O-rich environment. The molecular emission also shows other peculiarities like rotation, potential mass-transfer and voids, all which suggest that a companion may be present. We deduce the position angle and inclination of the symmetry axis of the system, and conclude that the observable constaints favour the hypothesis that the EDE is dominated by a tangential velocity field, as opposed to a radial one. In addition, we also present recent findings of the CSE of R Doradus. Preliminary observations with ALMA (cycle 2) at coarse spatial resolution reveal that the innermost regions exhibits a spatial offset between the red and blue shifted emission for most observed molecules. We crudely model the highly compact 28SiO emission and show that it can be explained by a tilted rotating disk, with a tangential velocity governed by angular momentum conservation, and a small outward velocity field. Both EP Aqr and R Dor are semiregular variable stars. In combination with the recent observations and modelling results of the compact, edge-on, differentially rotating disk surrounding the nearby semiregular AGB star L2 Puppis, the increasing observational sample is revealing a potential relation between the pulsational classification of M-type AGB stars and their circumstellar morphology.


Tracers of stellar mass loss: Mid-IR colors and surface brightness fluctuations

Gonzalez-Lopezlira, Rosa Amelia

I present integrated colors and surface brightness fluctuation magnitudes in the mid-IR, derived from stellar population synthesis models that include the effects of the dusty envelopes around TP-AGB stars. The models are based on the Bruzual & Charlot CB* isochrones; they are single-burst, range in age from a few Myr to 14 Gyr, and comprise metallicities between Z = 0.0001 and Z = 0.04._x000D_ I compare these models to mid-IR data of AGB stars and star clusters in the Magellanic Clouds, and study the effects of varying self-consistently the mass-loss rate, the stellar parameters, and the output spectra of the stars plus their dusty envelopes. I find that models with higher than fiducial mass-loss rate are needed to fit the mid-IR colors of ``extreme" single AGB stars in the Large Magellanic Cloud. Surface brightness fluctuation magnitudes are quite sensitive to metallicity for 4.5 um and longer wavelengths at all stellar population ages, and powerful diagnostics of mass-loss rate in the TP-AGB for intermediate age populations, between 100 Myr and 2-3 Gyr. Observations with the upcoming JWST should be able to resolve global changes in the mass-loss rate of a stellar population, and actually resolve whether a relation between metallicity and mass-loss exists.


On the silicate crystallinities of oxygen-rich evolved stars and their mass-loss rates

Jiang, Biwei

For decades ever since the early detection in the 1990s of the emission spectral features of crystalline silicates in oxygen-rich evolved stars, there is a long-standing debate on whether the crystallinity of the silicate dust correlates with the stellar mass-loss rate. To investigate the relation between the silicate crystallinities and the mass-loss rates of evolved stars, we carry out a detailed analysis of 28 nearby oxygen-rich stars. We derive the mass-loss rates of these sources by modelling their spectral energy distributions from the optical to the far-infrared. Unlike previous studies in which the silicate crystallinity was often measured in terms of the crystalline-to-amorphous silicate mass ratio, we characterize the silicate crystallinities of these sources with the flux ratios of the emission features of crystalline silicates to that of amorphous silicates. This does not require the knowledge of the silicate dust temperatures, which are the major source of uncertainties in estimating the crystalline-to-amorphous silicate mass ratio. With a Pearson correlation coefficient of -0.24, we find that the silicate crystallinities and the mass-loss rates of these sources are not correlated. This supports the earlier findings that the dust shells of low mass-loss rate stars can contain a significant fraction of crystalline silicates without showing the characteristic features in their emission spectra.


Alien visitors rule cosmic makeup

Decin, Leen

After decades of failures and misunderstandings, scientists have solved a cosmic riddle — what happens to the tons of dust particles that hit the Earth every day but seldom if ever get discovered in the places that humans know best, like buildings and parking lots, sidewalks and park benches. The answer? Nothing. Look harder. The tiny flecks are everywhere, all over the roof. The morphology of these flecks - micrometeorites - is a first hint of their extraterrestrial origin, the determination of their chemical makeup is the decisive making body. But how is that cosmic dust formed? What important clues on stellar winds are hidden in these extraterrestrial flecks? What do these microscopic samples supported by astronomical observations tell us about the future evolution of our own Sun? In this talk, I will discuss how interdisciplinary research linking astronomy and chemistry – `astrochemistry’ – can reveal the true formation pathways of these little grains.


Search for technetium and s-process elements in binary central stars of planetary nebulae

Löbling, Lisa

Low- and intermediate mass stars experience a phase of carbon enrichment and slow neutron-capture nucleosynthesis (s-process) on the AGB. One particularly interesting s-process element is the relatively short-lived (with a half-life of 211,000 years) technetium (Tc), whose presence in the atmosphere of a star is a clear indication that nucleosynthesis happened very recently. _x000D_ Analysing the element abundances, especially the enrichment of carbon and s-process elements not only in the hot evolved stars at the centre of planetary nebulae (CSPNe), which followed the AGB phase, help to derive constraints for the evolution of these CSPNe, i.e. the timescales of their formation. Doing so also in their companions if they are in a binary, provides information on the mass-transfer history and the post-common envelope evolution that led to the planetary nebula (PN)._x000D_ We present the study of the two PNe which were shown to harbour binary systems in their centres – in both cases the red giant companion is dominating in the visible. Using high-resolution spectra of these red giants, we analyse the presence of Tc and the abundances of carbon and s-process elements in their atmospheres – a clear sign of pollution by mass transfer from the former AGB-star – and compare the results to predictions from stellar models


A carbon star sheds its skin

Wittkowski, Markus

While the basics of this mass-loss process from AGB stars are understood, we are still investigating how it begins near the surface of the star. Here, we present recent near-IR H-band aperture synthesis images of the carbon AGB star R Sculptoris with an angular resolution of 2.5 mas obtained with the PIONIER instrument at the Very Large Telescope Interferometer. The data show a stellar disk of diameter ~9 mas exhibiting a complex substructure including one dominant bright spot. It has a peak intensity of 40% to 60% above the average intensity of stellar disk. This contrast is significantly higher than expected for photospheric convection, and we also expect the photosphere to be obscured by molecular and dusty layers of the atmosphere. The companion of R Sculptoris, as detected through previous ALMA images, is too far away to cause this complex substructure close to the surface. We interpret the complex structure as caused by giant convection cells, resulting in large-scale shock fronts, and their effects on clumpy molecule and dust formation seen against the photosphere at distances of 2-3 stellar radii. We obtained a Rosseland radius of 355 ± 55 Rsun, an effective temperature of 2640 ± 80 K, and a luminosity of log L/Lsun = 3.74 ± 0.18. These parameters match evolutionary tracks of initial mass 1.5 ± 0.5 Msun and current mass 1.3 ± 0.7 Msun. These constraints are important for the modelling of the mass loss history of the star and the interpretation of the spiral structure seen by ALMA at larger scales. Our visibility data are best fit by a model without a wind, which may point to problems with the wind models at low mass-loss rates._x000D_ I will also give an outlook to similar aperture synthesis imaging of red supergiants, including V766 Ven with its in-contact or common-envelope companion. I will discuss ongoing work to compare results from aperture synthesis imaging to 3D models.


Multiband study of AGB stars in clusters

Hojaev, Alisher S.

The interest to AGB population which consists of evolved cool luminous low- to intermediate mass stars (0.6-10 Ms) and exhibits the late phase of life of solar-type stars as well is still growing these years. In the one hand, increasing data stream from the modern facilities and instruments revealing new or clarifying known properties of AGB stars, in the other hand, enhancing computer capabilities allow to high-performance 3D modeling of physical conditions and phenomena connected with AGB stars. Especial importance of red giants observations in AGB and pre-AGB stars stages is in application of their group properties (particularly, TRGB and RCG) to the modeling of stellar systems, particularly open and globular clusters, for example, to estimate the age of the system. For this purpose we have carried out multiband photometry for the large sample of open clusters on 0.6-1.5 m class telescopes at Maidanak, Byurakan, Lulin observatories. The group parameters for clusters were determined and discussed. The continuation of observational study of AGB stars is planned. One of the new and perspective observational programs in research of open, globular clusters and other stellar systems is a intermediate and narrow band photometry within the frame of international all-sky survey of northern hemisphere. Further we intend to obtain spectral and deep photometric data using modern adaptive 4 m class telescope to be created in Uzbekistan and has been intended to install at Maidanak observatory well-known by excellent conditions for astronomical observations (large amount of clear sky, high quality and stability of atmosphere optics, etc) and advantageous location for time domain astrophysical research.


The longest period Long Period Variables in the Magellanic Clouds

Groenewegen, Martin

There are thousands of Long Period Variables known in the Magellanic Clouds mainly due to microlensing surveys like MACHO, EROS and OGLE. However these variability surveys in the optical miss the reddest stars on the asymptotic giant branch as high mass-loss rates will obscure them. We have used the multi-epoch nature of the VISTA Magellanic Cloud (VMC) survey in the K-band in combination with literature data to look for very red AGB star variables. We discuss the results of this program that has revealed several stars with Mira-like amplitudes and periods beyond 1000 days.


Looking for new water-fountain stars

Uscanga, Lucero

We present simultaneous observations of H2O and OH masers, and radio continuum at 1.3 cm with the Very Large Array towards four water-fountain candidates.  Water fountains (WFs) are evolved stars, in the AGB and post-AGB phase, with collimated jets traced by high-velocity H2O masers. Up to now, only 15 sources have been confirmed as WFs through interferometric observations. We are interested in the discovery and study of new WFs. A higher number of these of sources is important to understand their properties as a group, because they may represent one of the first manifestations of collimated mass-loss in evolved stars. These observations provide information about the role of magnetic fields in the launching of jets in WFs. Our aim is to ascertain the WF nature of these candidates, and investigate the spatial distribution of the H2O and OH masers. 


The temporal evolution of neutron-capture elements in the Galactic discs

Spina, Lorenzo

Important insights into the nucleosynthetic history of elements are contained in the chemical compositions of stars. In this talk I will review the results obtained from the high-precision abundance determinations of 12 neutron-capture elements over a sample of 79 solar twin stars. This valuable dataset allowed us to study the [X/Fe]-age relations over a time interval of 10 Gyr and among stars belonging to the Galactic thin and thick discs. These relations showed that the s-process has been the main channel of nucleosynthesis of n-capture elements during the evolution of the thin disc, while the thick disc is richer in r-process elements due to its rapid and intensive formation. In addition, abundances of the heavy (Ba, La, Ce) and light (Sr, Y, Zr) s-process elements revealed important details on the dependence between the yields of AGB stars and the stellar mass or metallicity.


ZOOMING INTO THE COMPLEX DUSTY ENVELOPES OF C-RICH AGB STARS

Lykou, Foteini

Recent advances in high-angular resolution instruments (VLT and VLTI, ALMA) have enabled us to delve deep into the circumstellar envelopes of AGB stars from the optical to the sub-mm wavelengths, thus allowing us to study in detail the gas and dust formation zones (e.g., their geometry, chemistry and kinematics). I will be reporting on the most recent developments of our study of 15 evolved stars in the near-infrared at high-angular resolution. The focus of the presentation will be on four (4) C-rich AGB stars from the initial sample. I will be presenting our multi-wavelength tomographic studies of the dusty layers of the circumstellar envelopes of these C-rich stars, i.e. the variations in the morphology and temperature distribution of the dust w.r.t. different phases, as well as discuss any hints on binarity from the interferometric data (Lykou et al., to be submitted).


Astrometric VLBI observation of the Galactic LPVs with VERA; Miras and OH/IR stars

Nakagawa, Akiharu

We present studies of LPVs in the Milky Way Galaxy (MWG) based on astrometric VLBI observations of H2O/SiO maser with VERA. The LPVs are 1 - 8 Msun AGB stars pulsating with typical period range of 100 - 1000 d. They are on the late stage of their life time, and show high mass loss ratio (10^-7 Msun/yr) before they evolve to planetary nebulae.LPVs in the LMC show some period-luminosity relations (PLR), and the PLR is used as distance indicator for Miras in the MWG. But metallicity is different between the LMC and MWG, it is also important to explore PLR of Miras in MWG. Since 2003, we have been observing dozens of Miras in MWG. The latest Galactic Mira’s PLR determined from our study is Mk = -3.52 logP+1.09 (+-0.14) (Nakagawa et al. 2016). In this conference, new results will be added. We find consistency of PLRs between LMC and MWG, but some stars show fainter magnitudes than expected from the PLR previously obtained. Though the discrepancy should be investigated, this can possibly indicate different properties of Miras in the LMC and MWG. Astrometric VLBI also has an advantage to reveal dynamical picture of circumstellar matters. A 3D model of circumstellar matter in “OH231.8+4.2” will be shown as an example.There are OH/IR stars showing quite long period longer than 1000 d (Extreme-OH/IR). They are thought to have initial masses of ~4 Msun and ages of ~10^8 yr. Recent studies predict galactic spiral arms bifurcating/merging in time scale of 10^8 yr. So, the Extreme-OH/IR stars can become a new probe to survey spiral arms. We started 43GHz VLBI observations of 2 Extreme-OH/IR stars, OH127.8+0.0 and NSV25875 from Oct. 2017. Images from phase-referencing analysis will be presented. Since evolutional relation between Miras and OH/IR stars is still an open question, sequential studies of LPVs along wide period axis are crucial. LPVs with longer period tend to be fainter in visible band, and VLBI observations play a promising and complementary role even in Gaia era.


AGB stars of the Magellanic Clouds as seen within the Delta-a photometric system

Paunzen, Ernst

About four decades ago, the Delta-a photometric system was introduced in order to investigate the flux depression at 5200A, typically for chemically peculiar stars of the upper main sequence. The a-index samples the flux of the 5200A region by comparing the flux at the center with the adjacent regions. The final intrinsic peculiarity index Delta-a was defined as the difference between the individual a-values and the a-values of normal stars of the same colour (spectral type). Here we present, for the first time, a case study to detect and analyse AGB stars in the Magellanic Clouds. For this, we use synthetic spectra and our photometric survey of the Magellanic clouds within the a-index. We find that AGB stars can be easily detected on the basis of their Delta-a index in an efficient way. We discuss our findings for AGB-stars of different chemistries.


Observational constraints on thermal pulses and the evolution of the mass-loss rate

Maercker, Matthias

AGB stars are the dominant producers of half of the elements heavier than iron, and thus play a crucial role in the chemical evolution of galaxies. The elemental yields from AGB stars depend critically on the number of thermal pulses the star experiences during AGB evolution, and hence on the lifetime of the star on the AGB. The evolution of the mass loss throughout the entire thermal pulse cycle therefore strongly affects the total return of elements and dust to the interstellar medium. ALMA observations of the detached shell around the carbon AGB star U Ant allow us to trace the mass-loss history since it was created in a thermal pulse 3000 years ago. In addition to the detailed structure of the shell, the ALMA data show the extended, smooth post-pulse and present day wind. Combined with Herschel/PACS image of thermal dust emission from the shell, the full analysis of the ALMA data will constrain the formation and evolution of the shell, and the evolution of the mass loss during and after a thermal pulse. The results are put into contrast to the detached-shell source R Scl, for which the evolution of the mass loss is significantly different from model predictions. Together with new ALMA data on the inner wind of R Scl, the results provide a unique test-bed for models of stellar evolution, and may be the only direct way of observationally constraining the evolution of AGB stars throughout the thermal pulse cycle. The observations of U Ant will eventually play a key role in our understanding of AGB stars and their place in the evolution of galaxies and the universe. 


Phase-lag measurements in dust-scattered, polarised light: the distance to R Scl

Maercker, Matthias

Distance measurements to astronomical objects are essential for understanding their intrinsic properties. For asymptotic giant branch (AGB) stars it is particularly difficult to derive accurate distance estimates. In the case of the carbon AGB star R Sculptoris, the uncertain distance significantly affects the interpretation of observations regarding the evolution of the stellar mass loss during and after the most recent thermal pulse. I provide a new, independent measurement of the distance to R Sculptoris, using phase-lag measurements in the thin, dusty detached-shell surrounding the star. By observing in polarised light, we imaged the shell in the plane of the sky, removing any uncertainty due to geometrical effects. The variation of the scattered light in the shell is of the same amplitude and period as the stellar pulsation, but with a phase lag that depends on the absolute size of the shell. The polarised, scattered light measurements reduce the absolute uncertainty of the distance estimate to this source to approx. 10%, giving a distance of 361 +/- 44 pc. This distance estimate is consistent with previous estimates, but is one of the most accurate measurements of the distance to R Scl to date. In the future, the same technique may be used for other detached-shell sources, or any circumstellar envelope with clear dust features, and may offer an independent distance measurement to AGB stars.


Modelling circumstellar gas and dust in Magellanic Cloud and Galactic carbon stars

Srinivasan, Sundar

Carbon is one of the most crucial products of AGB nucleosynthesis; AGB stars with initial masses in the range 1--4 Msun transport it into their outer layers via third dredge-up, eventually turning in C--rich AGB stars. After carbon monoxide, acetylene (C2H2) is one of the most abundant species in these shells, and it is the starting point for the synthesis of more complex hydrocarbons, and is also important for dust formation.Gas close to the photosphere is detected in near- and mid-IR spectra as deep absorption bands, which progressively weaken as emission from the increasing dust content fills in these features. The presence of C2H2 absorption on top of the dust continuum challenges this scenario however, suggesting that dust formation does not consume all available complex molecules, and that significant quantities of such molecules exist well outwards of the dust-formation radius. The absorption features are deeper at lower metallicities -- dusty Magellanic Cloud carbon stars show more prominent absorption than their Galactic counterparts, and simple models indicate a correlation between the dust-production rate and the column density of the circumstellar gas (e.g., Matsuura et al. 2006).We have analysed the Spitzer spectra of all the carbon stars in the Large and Small Magellanic Cloud, as well as in the Galaxy, by simultaneously modelling the dust spectrum and the gas absorption features. This allows us to investigate correlations between the various gas and dust parameters (e.g., the C2H2 column density and excitation temperature, and the dust-production rate) in a statistically significant sample. In addition, the large dataset allows us to probe any differences due to metallicity.


Carbon Enrichment in the Early Universe

Andersen, Johannes

Carbon-enhanced extremely metal-poor (CEMP) stars are our richest source of information on carbon (C) in the early universe. CEMP stars are of two basic varieties, with or without enrichment of elements synthesized by the slow neutron-capture process – CEMP-s and CEMP-no stars; each is associated with the inner and outer (most metal-poor; [Fe/H] < -3.5) halo populations, respectively. Canonically, CEMP stars were considered to be the result of local binary mass transfer, and much effort was invested in models according in this paradigm. However, Hansen et al. (2016a,b) showed that only ~80% of the CEMP-s stars are binaries, the CEMP-no stars and ~20% of the CEMP-s stars were single. Thus, C in these stars has been synthesised in a different object and implanted across interstellar space in the natal clouds of today’s CEMP stars, and both fast-rotating primordial stars and ‘faint’ supernovae have been invoked as possible progenitors. In a recent review, however, Höfner & Olofsson (2018) discuss whether combined radiation and pulsation pressures could eject C dust from a single star, obviating the need for a binary companion. Spectroscopic log g-s are uncertain for CEMP stars, so there is doubt if they are ‘normal’ giants or AGB stars. Gaia DR2 should yield actual log L values to tell, and the photometry should have sufficient precision and time coverage to confirm or refute the reality of the 3-400-day pulsations suggested by earlier radial-velocity work.


Modelling dust in the Nearby Evolved Star Survey (NESS) targets

Srinivasan, Sundar

The Nearby Evolved Stars Survey (NESS) is a multi-telescope project targeting a volume-limited (d < 2 kpc) sample of $\sim$400 evolved stars. NESS includes a 500-h ongoing JCMT survey of dust continuum as well as CO (2-1) and (3-2) line emission. The NESS sample includes XX oxygen-rich and YY carbon-rich AGB stars, as well as ZZ red supergiants, and many post-AGB stars and planetary nebulae. The NESS JCMT data facilitate the determination of the circumstellar dust distribution and estimation of the mass-loss history in the circumstellar shells, including any deviations from spherical symmetry (Dharmawardena et al., in prep). Radiative transfer models of this dust must update fits to mid-infrared spectral energy distributions (SEDs) to fit the far-IR and sub-mm information, and should reproduce the results from JCMT observations.In this poster, we present preliminary results of modelling dust in W Hya and U Ant. [W Hya information here]. NESS data has also revealed U Ant's detached shell for the first time in the sub-mm continuum. We first fit the mid-IR SEDs with models from the GRAMS grid (Sargent et al., 2011; Srinivasan et al., 2011), tacitly assuming spherical symmetry. Using these models as a starting point, we use the radiative transfer code 2Dust to explore ranges of parameters that will reproduce the radial profiles as determined by Dharmawardena et al. (in prep), including any evidence of detached shells and/or variable mass-loss rates.These models are the first step towards detailed modelling that incorporates data from optical through sub-mm SEDs and spectra, as well as other data such as interferometric visibilities. Modelling the large number of AGB stars targeted by NESS will lead to robust estimates of dust-production rates across the entire range of evolutionary stages along the AGB. Combined with modelling of the NESS CO line data, these can also be used to determine the gas-to-dust ratio throughout the circumstellar shell for the entire sample.


AGB star atmospheres modeling, as feedback to stellar evolutionary and galaxy models

Rau, Gioia

The chemical enrichment of the Universe is considerably affected by the contributions of Asymptotic Giant Branch (AGB) stars. These objects produce in their atmospheres heavy chemical elements, molecules, and dust, which, through the mass loss provided via their stellar winds, are placed into the interstellar medium. This talk will explore the modeling of cool, evolved stars, using ground-based interferometric facilities, and will introduce the capabilities of HST, and of future instruments onboard JWST.First, we will present our investigation of the O-rich AGB star R Peg in the near-IR with VLTI/GRAVITY (Wittkowski, Rau, et al., in prep.). We verified at different epochs the model-predicted variability of the visibility spectra, using CODEX model atmospheres for comparison with the observations. Our results, showing an anti-correlation between continuum radius and visual lightcurve, are consistent with 1D CODEX dynamic model atmosphere predictions.Second, following Rau et al. (2017), we present preliminary results on modeling with the grid of DARWIN dynamic model atmospheres, the atmosphere of V Oph, observed with VLTI/MIDI data, the (so-far) only known C-rich AGB star showing interferometric variability (Ohnaka et al. 2007). The comparison with the multi-shell model parameters by Ohnaka (2007) will provide additional constraints on C-rich AGB stars outer atmospheric composition. We will also introduce the capabilities of VLTI/MATISSE in advancing this work.Our work aims also to give feedback to stellar evolutionary models by comparison of our results to the evolutionary tracks by Marigo et al. (2013), to constrain stellar masses, C/O ratios, and ages.


SMA Spectral Line Survey of the Proto-Planetary Nebula CRL 618

Nimesh, Patel

Carbon-rich Asymptotic Giant Branch (AGB) stars are major sources of gas and dust in the interstellar medium. AGB stars remain in their evolutionary stage for 1 to 10 Myrs, during which they have very high mass loss rates that increase at the end. During the brief (~1000 yr) period in the evolution from the AGB to the Planetary Nebula (PN) stage there are dramatic changes in the morphology from nearly spherical symmetry, to bipolar, quadrupolar and more complex structures, with the development of both slow and fast (100 km/s) outflows. The molecular composition of these objects' cirumstellar envelopes  also evolves from being similar to that of parent AGB star (mainly diatomic and small polyatomic species), to more complex molecules (including ions).We have started an observational study of a sample of Proto-Planetary nebulae (PPN) with the Submillimeter Array (SMA) to carry out spectral-line surveys of ~60 GHz frequency coverage in the 345 GHz band (similar to our published IRC+10216 line survey of 2011). Here we present results from the spectral line survey of the carbon-rich PPN CRL 618, covering a frequency range of 281.9 to 359.4 GHz. Observations were carried out in January 2016, September 2017, and November 2017,  with the SMA in compact (3" angular resolution), very extended (0.5”), and extended (1.0") configurations, respectively.More than 1100 lines were detected in CRL 618. The majority of them can be attributed to HC3N and c-C3H2, and their isotopologues. About 350 lines are as yet unassigned. The continuum emission is unresolved even at 0.5" resolution. Several hydrogen recombination lines are detected from the central HII region. Lines of CO, HCO+, CS show the fast outflow wings, while the majority of molecular emission arises from a compact region of about 1" diameter. We present LTE modeling and rotation temperature diagram analysis of HC3N, c-C3H2, CH3CN, and their isotopologues.


The Nearby Evolved Stars Survey: Project description and initial results

Scicluna, Peter

Outflows of asymptotic giant branch (AGB) and red supergiant (RSG) stars drive the chemical evolution of galaxies in the local Universe. Thanks to Spitzer, a number of Local Group galaxies have been observed in detail, allowing the dust-production rates of all AGB stars and RSGs to be determined, and hence the total dust injection rate for the galaxies. However, measuring the gas mass-loss rates for stars outside our galaxy is prohibitively difficult, making it unlikely that both components of mass loss can be studied for a large sample in the foreseeable future beyond our galaxy. Such systematic studies in the Milky Way remain conspicuously absent. The Nearby Evolved Stars Survey (NESS) aims to fill this gap, through a large multi-telescope project targeting a volume-limited sample of approximately 400 evolved stars within 2 kpc. We intend to derive the dust and gas return rates in the Solar Neighbourhood, and to constrain the physics underlying these processes. So far, the project includes an ongoing JCMT survey of CO (2-1), (3-2) and dust continuum towards the Northern and equatorial parts of the sample, an APEX programme to observe the Southern sources, and a pilot program at the Nobeyama observatory to observe CO (1-0). The key science goals of NESS include determining the total gas- and dust-mass returned to the local interstellar medium, constraining the onset of dust- and pulsation-driven winds, constraining mass-loss variability, gas-to-dust ratios, and dust properties in the outflows of evolved stars, measuring the 13C/12C ratio from CO isotopologues to constrain nucleosynthesis and stellar evolution, and statistical studies of Galactic AGB stars as a population. I will present a detailed description of the project, including the motivation and strategy, and highlight some of our early results. I will also briefly introduce the tools we are developing that will, along with all raw and reduced NESS data, be released to the community to aid reproducibility.


Observing the mass-loss of nearby red supergiants through high-contrast imaging

Scicluna, Peter

Mass-loss in cool supergiants remains poorly understood, but is one of the key elements in the final stages of their evolution, helping to determine when a given star will explode as a supernova, and what kind of supernova it will become. Some show evidence of asymmetric mass loss, discrete mass-ejections and outbursts, with seemingly little to distinguish them from more quiescent cases. Various explanations have been put forward, including multiplicity, magnetic effects and starspots, but consensus has yet to emerge. To explore the prevalence of discrete ejections and companions we have conducted a high-contrast survey using near-infrared imaging and optical polarimetric imaging of nearby southern and equatorial red supergiants, using the extreme adaptive optics instrument SPHERE, which was designed to image planets around nearby stars. I will present a highly-automated pipeline designed to reduce this data and the initial results of the survey, including the detection of large (approximately 0.5 um in radius) dust grains in the ejecta of VY CMa and a candidate dusty torus aligned with the maser ring of VX Sgr, in which there are also suggestions of large dust grains. These will be compared with observations of circumstellar material at other wavelengths. I will briefly speculate on the consequences for our understanding of mass loss in these extreme stars.


Stacking analysis of the HERITAGE data to statistically study far-IR dust emission from evolved stars

Dharmawardena, Thavisha

By fitting the mid-IR SEDs of evolved stars in the LMC and SMC with GRAMS grid of radiative transfer models (Sargent et al., 2011; Srinivasan et al., 2011), Riebel et al., 2012 and Srinivasan et al., 2016 has estimated the dust budget in these galaxies. These fits may not account for cold dust, emitting in the far-IR, and may thus underestimate the dust-production rate (DPR). Herschel data can be analysed for cold dust emission in excess of the predictions from fits to the mid-IR SEDs (Boyer et al., 2012). However, due to confusion with background emission from the interstellar medium (ISM) and Herschel’s limited  spatial resolution only 35 of the tens of thousands of Spitzer-identified evolved stars in the LMC and SMC have identified counterparts in the Herschel HERITAGE point source catalogue (Jones et al., 2015).In order to determine the presence of a cold dust component in a statistical sense, we revisit the Herschel HERITAGE data of the LMC and SMC using a method of image stacking similar to what is commonly used in extragalactic observational studies (e.g. Schreiber et al., 2015). We improve the signal-to-noise of the cold dust emission by using postage-stamp sized chunks of the Herschel HERITAGE maps and co-adds the cutouts to derive the photometric flux at each wavelength for the sources included in the co-added stack.Results of the stacking analysis can be used to detect the presence of an additional dust component, and therefore refine the dust budget estimates of Riebel et al., 2012 and Srinivasan et al., 2016 for the same sample. They will also allow us to test the predictions of FIR flux by the GRAMS models. The SED of the average additional cold dust component can then be analysed to adjust the average dust-production rate.With the tens of thousands of evolved stars at our disposal, we can divide the sample into sub-categories, such as by chemistry, mass-loss rates, initial mass for which to investigate the presence of cold dust.


Measuring spatially resolved gas-to-dust ratios in AGB stars

Wallstrom, Sofia

Asymptotic Giant Branch (AGB) stars form significant circumstellar envelopes of gas and dust, but the relation between these two components is not well understood. The gas-to-dust ratio across the envelope carries vital information about, e.g., the dust-condensation efficiency, and is often used to assume a gas mass from a measured dust mass and vice versa. A canonical value of 100 or 200 is often used for Galactic stars. Knapp (1985) measured gas-to-dust ratios in Galactic evolved stars -- finding ratios around 160 for O-rich stars, and 400 for C-rich stars -- which remains the only direct measurement of this ratio for evolved stars in the Solar Neighbourhood. We have directly measured gas and dust masses for a sample of 15 nearby AGB stars, using JCMT CO-line and continuum observations. This serves as a pilot study for the Nearby Evolved Stars Survey (NESS; PI: P. Scicluna) project which will provide similar observations of ~400 AGB stars in a volume-limited sample out to 2 kpc.The addition of submm continuum data to existing IR photometry allows us to constrain the cold dust component and hence calculate dust masses from near-IR to submm SED fits (Dharmawardena et al., in prep). The continuum observations are matched with multi-pixel CO 3-2 observations and mapping observations of CO 2-1, providing spatially resolved gas-to-dust ratios. We derive gas masses by fitting the CO emission with an improved version of the line radiative transfer code presented by Kemper et al. (2003). It now includes a sophisticated algorithm to explore parameter space and find the best fit model to the observations and robust statistical uncertainties. The model also provides results at a range of impact parameters throughout the circumstellar envelope and can correctly account for data from different telescopes, both single-dish and interferometric, allowing us to fit spatial information along with the line profiles. This code will be publicly released as part of the NESS project.


Properties of OH/IR stars versus YSOs in mid- and far-infrared surveys

Loup, Cecile

To disentangle very optically thick AGB stars and young stellar objects in infrared photometric surveys has always been a challenge. The typical double-peaked OH maser profile of OH/IR stars proves that they are AGB or post-AGB stars, while the detection of a CH3OH maser signs the presence of YSOs, independently of any infrared observation. About 2000 OH/IR stars and 500 YSOs with CH3OH maser  emission have been identified in the AKARI-IRC/FIS, MSX6C, GLIMPSE, MIPSGAL, and Hi-Gal surveys. It is shown that OH/IR stars can be as red as YSOs in the mid-infrared, with m(8)-m(24) reaching 10. OH/IR stars and YSOs with m(8)-m(24) > 2.5 cannot be disentangle with mid-infrared observations only. On another hand, the combination of mid- and far-infrared observations allows to set up selection criteria to disentangle OH/IR stars and dusty YSOs. These criteria have been applied to the "intrinsically red sources in the galactic midplane" selected by Robitaille et al. (2008). It is shown that these "red sources" towards the galactic plane are mostly AGB stars rather than YSOs, with proportions of the order of 70% versus 30%, respectively.


The abundance of SiC2 in Carbon Star Envelopes

Massalkhi, Sarah

During the late stages of their evolution, AGB stars experience significant mass loss, which result in circumstellar envelopes (CSEs). These environments are efficient factories of molecules and dust grains. Although much has been advanced recently, there is still much to understand about how dust grains are formed and what the main gas-phase seeds are. This is the main driver of the ERC Synergy Project NANOCOSMOS. Silicon carbide (SiC) dust grains are exclusively found in the envelopes around C-type (C/O>1) AGB stars (Treffers & Cohen 1974). Here, we explore what the main precursor seeds of SiC dust grains are. Only three gas-phase molecules containing the Si-C bond have been observed in C-rich envelopes around AGB stars. The ring molecule SiC2 has been observed towards a few AGB and post-AGB stars (Thaddeus et al 1984; Bachiller et al. 1997; Zhang et al. 2009a,b), while SiC and Si2C have been only observed in the C star envelope IRC +10216 (Cernicharo et al. 1989, 2015). Much of the knowledge about the role of these three molecules as seeds of SiC dust grains comes from the study of IRC +10216. The scenario emerged from these studies suggests that only SiC2 and Si2C are present in the inner circumstellar layers of IRC +10216, while SiC is probably a photodissociation product of these molecules, and thus it is restricted to the outer envelope. This scenario indicates that SiC2 and Si2C are likely the main gas-phase seeds to form SiC dust grains. To explore the role of gas-phase SiC2 molecules on the formation of silicon carbide dust, we have used the IRAM 30m telescope to observe SiC2 in a wide sample of C-rich AGB stars. The observations have been interpreted carrying out non-LTE excitation and radiative transfer calculations to estimate the fractional abundance of SiC2 in the CSEs. The behavior of the abundance of SiC2 as a function of the envelope density indicates that this gas-phase molecule does indeed play an important role as a seed of silicon carbide dust.


Using Gaia to measure dynamics in AGB stars

Chiavassa, Andrea

Convection in AGB stars is a difficult process to understand because it is non-local and three-dimensional, and it involves non-linear interactions over many disparate length scales due to the low gravity and corresponding large pressure scale heights. In this context, global numerical hydrodynamical simulations of surface convection carried out with CO5BOLD are essential to a proper and quantitative analysis of the observations. Moreover, a grid of global models of the outer convective envelope and the inner atmosphere of AGB stars is now available (Freytag et al. 2017, A&A, 600, 137). I will present the benefit of the employment of these simulations to produce spectrophotometric and interferometric observables in the framework of next generation instruments (e.g., ESPRESSO@VLT and MATISSE@VLTI).


Rotating stellar models of low- and intermediate-mass stars from the pre-main sequence up to the AGB

Mendes, Luiz Themystokliz Sanctos

We report on rotating stellar evolutionary models of low- and intermediate-mass stars computed with the ATON 2.3 stellar evolution code. Rotation has been implemented by using the Kippenhahn & Thomas (1970) method in the approximation by Endal & Sofia (1986), which relies on the deformation of the star's equipotential surfaces due to rotation provided that the total (gravitational plus rotational) potential is conservative. This method was already used in the ATON 2.3 code in the context of low-mass stars during the pre-main sequence and main-sequence stages, and now is being extended to intermediate mass stars and evolution stages up to the asymptotic giant branch (AGB). Some preliminary results about the evolution of some typical rotating models in the range of 1 to 5 solar masses from the pre-main sequence to the AGB are presented and discussed.


Metallic Line Doubling in the Spectra of the Variable R Scuti Star

chafouai, khadija

We conducted an observation campaign of the brightest and the most irregular RV Tauri star R Scuti. This observation campaign spanned over 26 days from June to November 2016 at the observatory in thehigh Atlas Mountains, at an altitude of 2700 m, in Morocco. The instrument set up includes two telescopes mounted on each other. For spectroscopy, a C14 was used along with an eShel spectrograph. Forphotometry, ASA 10 N was used with a QSI CCD camera were used with various color filters (U-B- V). High resolution spectra (R ˜ 12 000) obtained between 4289 Å and 7125 Å. In this work, we present theobservation of line doubling absorption in the spectra of the R Scuti variable star on the metal lines of FeI (?6546.245 Å, 6569,224 Å and ?6750.146 Å), and Ti II(?5188.7Å) around the phases ? = 1.18 and ? =1.70. We observed the splitting in the form of broadening of lines on the Ti I (?5866.46Å) in the same phases. This metal lines show two transitions, from the red component to the blue component and vice versa.


Infrared light curves of dusty & metal-poor AGB stars

Goldman, Steve

The effects of metallicity on both the dust production and mass loss of evolved stars have consequences for stellar masses, stellar lifetimes, the progenitors of core-collapse supernovae, and the origin of dust in the ISM. With the DUST in Nearby Galaxies with Spitzer (DUSTiNGS) survey, we have discovered samples of dusty evolved AGB stars out to the edge of the Local Group, reaching metallicities down to 0.6% solar. This makes them the nearest analogs of AGB stars in high-redshift galaxies. We present new infrared light curves of the dustiest AGB stars in 10 galaxies from the DUSTiNGS survey and show how the infrared Period-Luminosity (PL) relation is affected by dust and by metallicity. These results have implications for the efficiency of AGB dust production at high-redshift and for the use of the Mira PL relation as a potential distance indicator. 


Mid-IR spectroscopic observations of the dustiest AGB stars in the Galaxy

Goldman, Steve

We have used the VISIR spectrograph at the Very Large Telescope to target 21 of the most luminous and heavily-obscured oxygen-rich evolved AGB stars in the galaxy. Low resolution N-band (8 - 13 um) spectroscopy was used to target the 10 µm silicate feature. The sample, with a median luminosity of ~10,000 solar luminosities and a median mass loss rate of ~10-4 solar masses per year, has shown higher mass loss rates than previous Galactic and Large Magellanic Cloud samples, given their luminosities. These results, along with expansion velocities from previous OH maser detections, have been used to test and refine the wind-driving and mass loss mechanisms. Our new spectra have also allowed us to study the dust composition and geometries of these sources. 


Magritte: a new multidimensional accelerated general-purpose radiative transfer code

De Ceuster, Frederik

Radiative transfer plays a key role in the dynamics, the chemistry and the energy balance of all kindsof astrophysical objects. It provides the radiative pressure to drive stellar winds, it affects the chemistrythrough various photoionization and photodissociation reactions, and it can very efficiently heat or coolvery specific regions in the wind. Therefore it is essential in modelling these objects to properly accountfor all radiative processes and their interdependence. This however can be complicated by i) an intricate3D geometrical structure shielding or exposing specific regions to radiation, ii) the scattering by dust orfree electrons, and iii) the mixing in frequency space due to Doppler shifts caused by velocity gradientsin the medium. The tight coupling between radiative transfer and the often very specialized and diversedynamical and chemical models furthermore requires a modular code that can easily be integrated.To address these needs we will present Magritte: a new multidimensional accelerated general-purposeradiative transfer code. Magritte is especially designed to have a well scaling performance on variouscomputer architectures. Appended with a dedicated chemistry and thermal balance module it can self-consistently calculate the temperature field, chemical abundances and level populations. Magritte is adeterministic ray-tracing code that obtains the radiation field by solving the transfer equation along afixed set of rays originating from each grid cell. It iteratively accounts for scattering and treats the fullfrequency space. This allows us to self-consistently model the chemistry and energy balance in stellarwinds and perform synthetic observations. We will apply Margritte by post-processing snapshots ofhydrodynamical wind and bowshock simulations and will present its integration in self-consistent hydro-chemical AGB wind models.


Using Blue Straggler Binaries to Constrain AGB Mass Transfer

Gosnell, Natalie

Complete membership studies of open star clusters reveal that 25% of the evolved stars follow pathways in stellar evolution that are impacted by the presence of binary stars. Recent studies show that the majority of blue straggler stars, traditionally defined to be stars brighter and bluer than the corresponding main sequence turnoff, are formed through mass transfer from an Asymptotic Giant Branch (AGB) star onto a main sequence companion. We will present Hubble Space Telescope COS ultraviolet spectroscopy of WOCS 4540, a blue straggler-white dwarf binary in open cluster NGC 188 that formed through mass transfer from an AGB star onto a main sequence companion. The current cooling temperature of the white dwarf sets the timeline for the mass transfer history, and the mass of the white dwarf tells us the core mass of the AGB star at the end of mass transfer. The system must have experienced at least 60% mass transfer efficiency to create the blue straggler mass seen today. The existence of this binary in a well-studied cluster environment provides a unique opportunity to constrain both the pre- and post-mass transfer systems, improving our understanding of mass transfer and the interplay of AGB stars and binary dynamics.  


M1-92 revisited: the chemistry of an ejected common envelope

Alcolea, Javier

M1-92, Minkoswi's Footprint, is a 0.9 Mo pPN around a B-type post-AGB source. Previous IRAM Plateu de Bure 13CO 2-1 observations showed that this nebula is the result of a brief but huge mass-loss event that occurred 1200 yr ago. Using the IRAM 30m MRT, we have have conducted a very sensitive spectral line survey of M1-92, the observations almost fully covering the 3mm, 2mm, and 1.3mm bands. We have detected, for the first time in this source, numerous molecular species such as HCN, HNC. CN. CS, NS, SiO, SO, SO2, H2S, including the cations HCO+, N2H+, and SO+. These species include tracers of PDR chemistry and of shock chemistry, indicating that both processes are presently active in the source. More recently, some of the detected lines have been observed with sub-arc-second resolution, using the IRAM NOEMA 9-element array. Combining single dish and (old and new) interferometric data, we have determined the abundance of the most relevant species in the different parts of the pPN. Most relevant, C17O and C18O observations suggest a very shocking result: the derived [17O]/[18O] ratio indicates that, according to standard nucleo-synthesis models, the initial mass of the central star was 1.8 Mo. This is in agreement with the high mass of the nebula, but in contrast with the relatively low [12C]/[13C] ratio and the O-rich chemistry of the source. We suggest that all these results can be reconciled if we assume that the copious mass loss event occurred 1200 yr ago, which resulted in the formation of the present nebula, was also responsible for the premature ending of the AGB phase of the star. This would have prevented the 3rd dredge-up to occur, and so the source would have escaped from its transformation into a carbon rich source. If this hypothesis this is correct, and depending on the prevalence of such events, we may have to revise our knowledge on stellar yields from AGB stars.


Using barium stars to constrain binary interaction in systems containing stars with convective envelopes

Escorza, Ana

It is estimated that about half of the stars in the Galaxy have a companion which, at specific stages, can strongly affect their evolution. A rich zoo of objects can emerge from these interactions, ranging from thermonuclear novae, supernovae type Ia, or gravitational wave sources, to less energetic systems such as sub-dwarf B stars, barium stars or cataclysmic variables. Therefore, binary interaction plays a fundamental role in understanding stellar evolution and the chemical evolution of the Galaxy.During evolution, interactions increase when one of the components evolves to giant dimensions. We focus on chemically peculiar barium (Ba) stars, which are main-sequence or giant stars formed due to binary interaction when a former AGB companion (now a dim white dwarf) polluted them with enriched s-process matter. A long-standing problem, however, is that their observed orbital properties cannot be accounted for by models, so their formation is not well understood. Additionally, a second phase of interaction may occur when the Ba dwarf climbs the red giant branch (RGB), complicating this picture.In this contribution, we present new binary evolution models computed with the state-of-the-art code BINSTAR investigating the change of the orbital elements of Ba stars during their ascent of the RGB. We have accumulated large statistics for these systems: more than 400 Ba stars have recently been placed on the HR diagram, orbital elements for many systems have been accurately determined, and we expect a natural link between Ba dwarfs and Ba giants. Using information from dwarf systems, we have created a grid of models with different initial conditions (eccentricities, periods, masses, wind prescriptions and tidal efficiencies) to explore possible interaction mechanisms that would lead to the formation of the Ba giant orbits that we currently observe. Our ultimate goal is to obtain a better quantified understanding of binary interaction physics with convective primaries.


On the nature and mass loss of Bulge OH/IR stars

Groenewegen, Martin

We report on the succesful search for CO (2-1) and (3-2) emission associated with OH/IR stars in the Galactic Bulge. We observed a sample of 8 extremely red AGB stars with the APEX telescope and detected seven. The sources were selected at sufficient high galactic latitude to avoid interference by interstellar CO, which hampered previous studies of inner galaxy stars._x000D_ To study the nature of our sample and the mass loss we used VizieR and further literature study to collect photometric data and Spitzer IRS spectroscopy to construct the SEDs. In a first step we apply radiative transfer modelling to fit the SEDs and obtain luminosities and dust mass loss rates (MLR). Through dynamical modelling we then retrieve the total MLR and the gas-to-dust ratios. We derived variability periods of our stars from the VVV and WISE surveys. The luminosities range between approximately 4,000 and 5,500 Lsun and periods are below 700 days. The total MLR ranges between 10-5 and 10-4 Msun/yr. Comparison with the Vassiliadis & Wood (1993) evolutionary models shows that the progenitor mass is approximately 1.5 Msun, similar to the Bulge Miras and are of intermediate age (3 Gyr). If more massive OH/IR stars are rare in the Bulge this may explain the scarcity of Bulge carbon stars. The gas-to-dust ratios are between 100 and 400 and are similar to what is found for OH/IR stars in the galactic Disk. We will discuss the nature of this OH/IR population in view of the inner Galaxy stellar populations._x000D_ One star, IRAS 17347-2319, has a very short period of approximately 300 days which may be decreasing further. It may belong to a class of Mira variables with a sudden change in period as observed in some Galactic objects. It would be the first example of an OH/IR star in this class and deserves further follow-up observations.


Modelling the extended dust emission around carbon stars observed by Herschel

Mecina, Marko

We give an overview of Herschel PACS far infrared imaging observations of carbon stars and their extended envelopes. Dust structures known from previous infrared missions or expected because of molecular line emission observations are resolved in unprecedented detail. In particular, we study spherically symmetric detached shells. Some of these geometrically thin structures are also discovered in thermal dust emission around stars where there has been no detection so far. The associated stars show deep spectral features due to carbon bearing molecules, indicating a (very) high C/O ratio. This suggests that these objects are all in a very similar, late stage of the AGB, having undergone multiple dredge ups.The remarkable spherical symmetry justifies a straightforward application of 1D models to constrain the properties of the dust envelope, whose modulation in density is a consequence of short epochs of highly increased mass loss and/or wind-wind interaction between outflows of different velocity. We thus perform 1D dust radiative transfer calculations, first based on a parametrised density distribution, and in a more sophisticated approach on a combination of stationary wind models. The spatial information of PACS imaging provides crucial constraints. Emphasis is put on evaluating various dust grain setups. We show how the different grain parameters as well as the degeneracy between them affect the models and to which extent this prohibits accurate determination of the shell characteristics.


Imaging Red Supergiants with VLT/SPHERE/ZIMPOL

Cannon, Emily

In the red supergiant (RSG) phase of evolution massive stars show powerful stellar winds, which strongly influence the supernova progenitor properties and control the nature of the compact object that is left behind. Material that is lost in the stellar wind, together with that ejected in the final core collapse, contributes to the chemical enrichment of the local interstellar medium. The mass-loss properties of RSGs are however poorly constrained. Moreover, little is known about the wind driving mechanism. To provide better constraints on both mass-loss rates and physics, high angular resolution observations are needed to unveil the inner regions of the circumstellar environment, where the mass loss is triggered.Using the VLT-SPHERE/ZIMPOL adaptive optics imaging polarimeter, spatially resolved images of a small sample of nearby RSGs were obtained in multiple filters. From these data, we obtain information on geometrical structures in the inner wind, the onset radius and spatial distribution of dust grains, and dust properties such as grain size. As dust grains may play a role in initiating and/or driving the outflow, this could provide us with clues as to the wind driving mechanism.


The Structure of the Inner Circumstellar Shell in Miras

Hinkle, Kenneth

Stars undergo most of their mass loss on the AGB.  While the overall process is understood many details remain that need further study.  We report on spectra of near-infrared, low-excitation CO fundamental and first-overtone lines in three Miras.  The observations are in time series spanning up to two decades.  The temperature and velocity probed by the low-excitation CO lines are significantly different than that of the stellar photosphere measured by higher excitation lines.  The low-excitation CO region undergoes aperiodic variations.  The lines are shown to probe the structure of the circumstellar shell close to the star.  By drawing from the extensive literature on multi-frequency observations of Miras we show how the low-excitation CO region is connected to the structure of the circumstellar region and the mass loss process.


On cylindrically symmetric solutions of polarized radiative transfer equation

Freimanis, Juris

While modelling astronomical objects in general, and envelopes of AGB stars in particular, only numerical solutions of radiative transfer equation (RTE) can give realistic pictures of real objects. However, analytic solutions of RTE (and other equations of mathematical hysics) for simplified, symmetric model cases are desirable as tests of correctness and precision of computer programs. Besides, sometimes astronomical objects can be looked upon as approximately having some particular symmetry, and analytic solution of RTE can be used for evaluation purposes.Continuum radiation from dust envelopes of AGB and post-AGB stars is often polarized due to form and morphology of the envelope, orientation of dust grains and asymmetry of the primary sources of radiation. Polarization gives valuable astrophysical information, and solution of RTE without accounting for polarization is basically incorrect.Some exact model solutions of monochromatic polarized RTE in homogeneous isotropic mirror-symmetric medium, in case if the radiation field has cylindrical symmetry, are reviewed in this report. Cylindrically symmetric Green's function for infinite medium is mentioned, stressing its complexity. Cylindrically symmetric inner eigenfunctions of RTE are presented as much simpler alternative, which can be used if the scattering medium (e.g. dust cloud) is a homogeneous cylinder of infinite length but finite radius, and the medium is irradiated from outside. It is proven that cylindrically symmetric homogeneous polarized RTE has solutions (eigenfunctions) indexed by asymptotic azimuthal index (AAI) adopting arbitrary integer values. There are at least one convergent inner eigenfunction and one divergent formal outer eigenfunction for each value of AAI; clear analytic expressions are given for them. If the modulus of AAI does not exceed 3 then these are the only eigenfunctions of cylindrically symmetric RTE.


Carbon and oxygen isotopes in AGB stars, from the cores of AGB stars to presolar dust

Lebzelter, Thomas

Isotopic ratios are a powerful tool for gaining insights into stellar evolution and nucleosynthesis. The isotopic ratios of the key elements carbon and oxygen are perfectly suited to investigate the pristine composition of red giants, the conditions in their interiors, and the mixing in their extended atmospheres. Of course the dust ejected from red giants in their final evolution also contains isotopically tagged material. This red giant dust is present in the solar system as presolar dust grains. Thus evolved star C and O isotope ratios can be measured in two ways, spectroscopically in the atmospheres of red giant stars and in grains in the laboratory.We have measured isotopic ratios of carbon and oxygen in spectra from a large sample of AGBstars including both miras and semiregular variables. We show how the derived ratios compare with expectations from stellar models and with measurements in presolar grains. We will show how the isotopes measure the initial stellar masses of AGB stars. Comparison of isotopes that are affected by different types of neuclosynthesis provides insights into galactic evolution.


Circumstellar chemistry of Si-C bearing molecules in the C-rich AGB star IRC+10216

Velilla-Prieto, Luis

A significant contribution to ISM dust enrichment is produced by AGB stars, where nucleation of grain seeds occurs near the photosphere. Dust formation and growth is far from being well understood because there are many unknowns in the formation pathways; the condensation sequences of refractory species; and the dependence on stellar and circumstellar properties.Silicon carbide together with amorphous carbon are the main components of dust grains in the atmospheres of C-rich AGB stars. Therefore, the study of gas phase carriers of Si-C bonds in the envelopes of C-rich stars is a promising approach for shedding light on the formation of SiC dust. Small gaseous Si-C bearing molecules (such as SiC, Si2C, and SiC2) are efficiently formed close to the stellar photosphere. They likely condense onto dust seeds owing to their highly refractory nature at the lower temperatures (i.e., below about 2500 K) in the dust growth zone which extends a few stellar radii from the photosphere. Beyond this region, the abundances of Si-C bearing molecules are expected to decrease until they are eventually reformed in the outer shells of the circumstellar envelope, owing to the interaction between the gas and the interstellar UV radiation field.Our goal is to understand the time-dependent chemical evolution of Si-C bond carriers probed by molecular spectral line emission in the circumstellar envelope of IRC+10216 at millimeter wavelengths. Here, we will present our latest results derived from the analysis of single-dish (IRAM-30m) and interferometric data (SMA, NOEMA, and ALMA) of Si-C bearing molecules in the circumstellar envelope of this prototypical AGB star.


Variability in Post-AGB Stars: Pulsation and Binarity in Proto-Planetary Nebulae

Hrivnak, Bruce

We have intensely monitored photometric variability in proto-planetary nebulae (PPNe) over the past 25 years and radial velocity variability over the past ten years.  Pulsational variability has been obvious, in both the light and velocity, although the resulting curves are complex, with multiple periods and varying amplitudes.  Nevertheless, consistent patterns have emerged that give insight into the structure and evolution of the stars.  Observed periods range from 25 to 160 days, and the periods and amplitudes reveal evolutionary trends.  Evidence for binarity is more elusive, with no clear evidence emerging from these long-term radial velocity or light curves. We will present our observational results to date for approximately 30 PPNe, and discuss these results, including the search for period changes that might help constrain post-AGB evolutionary timescales. 


The role of asymptotic giant branch stars in the chemical evolution of the Galaxy

Tautvaisiene, Grazina

Asymptotic branch stars play an important role in enriching galaxies by s-process elements. Recent studies showed that their role in producing s-process elements in the Galactic disk was underestimated and should be reconsidered. Based on high-resolution spectra obtained with the ESO VLT UVES and 2.2 m MPG/ESO FEROS spectrographs, we have determined abundances of neutron-capture elements in a sample of more than 300 stars located in the field and open clusters towards the Galactic centre and investigated an elemental enrichment pattern according to their age and galactocentric distance. 


Monitoring of the AGB star R Doradus using SPHERE/ZIMPOL

Khouri, Theo

In recent years, our understanding of the inner environment of evolved stars has been reshaped by high-angular-resolution imaging and the development of very sensitive instruments. These have revealed the highly dynamical inner circumstellar environments, from where the outflows are accelerated, and even details of the stellar photospheres, with overall asymmetries, hotspots, and shaping by convective cells. The new observational constraints are very important to guide the ongoing development of advanced computer simulations of convection, wind driving, and binary interactions. We have used the extreme adaptive optics and polarimetric imager SPHERE/ZIMPOL to monitor the AGB star R Doradus in visible wavelengths at 12 epochs over eight months using up to five filters per epoch. The stellar disc and the dust-formation region are resolved in the images obtained. This allows us to study the processes shaping the stellar disk and the time variability of dust formation. R Dor varies significantly in size between the different filters. Nonetheless, the images in each epoch reveal mostly a consistent picture of an asymmetric stellar disk with features that evolve on timescales from a few weeks to a few months. The polarised light images are interpreted using radiative transfer models and show a significant variation in the total amount and in the distribution of the dust grains. By analysing the wavelength dependence of the scattered light, we also study how the properties of the dust particles vary between the different epochs of observation.


Observational Properties of Miras in the KELT Survey

Arnold, Robert

We present a catalog of the observed properties of Mira variable stars detected with the KELT survey.  Asymptotic giant branch (AGB) stars were identified in KELT using their 2MASS and WISE colors, and their variability properties were determined from KELT data. Of the 21279 KELT AGB objects, over 700 Mira variables were identified.  Here we present their observed periods, luminosities, and temperatures that will place important constraints on future theoretical work on the effect convection has on pulsation periods and stability.


Binary evolution and double sequences of blue stragglers in globular clusters

Jiang, Dengkai

Double sequences of blue stragglers in the color-magnitude diagram of globular clusters are important to study the formation mechanisms of blue stragglers. We study the formation of blue stragglers from binary evolution. We find the location of a blue-straggler binary produced by mass transfer in binary systems depends on the contribution of the mass donor to the total luminosity of the binary, because two components of this binary system cannot be distinguished. In the blue sequence, the blue-straggler binaries have a blue straggler orbiting a cool white dwarf. However, the red sequence include blue-straggler binaries that are experiencing mass transfer, or just terminating mass transfer, and the blue-straggler binaries that the blue stragglers have evolved away from the blue sequence. We suggest that mass transfer in binary systems can contribute to the blue stragglers in both of the sequences in globular clusters. 


Jet formation revealed in post-AGB binaries

Bollen, Dylan

The presence of a binary companion is believed to be the main shaping agent for asymmetric Planetary Nebulae (PNe). The binary interactions induce jet formation which drives the non-spherical structures that we observe in PNe. This is an important feedback mechanism of dust and gas to the galaxy.  However, the central regions of PNe are highly obscured. For this reason, we turn our focus to the PN-progenitors, namely post-Asymptotic Giant Branch (post-AGB) binaries. In our recent work, we find that the inner regions of these objects can be observed directly, making them important tracers for jet formation in evolved binary systems. In this talk, I will present our quantitative study on the origin and properties of high-velocity outflows in post-AGB binaries. By comparing a geometrical model of these systems with the observational data, we were able to deduce the jet geometry which revealed that the gaseous circum-companion disk is the origin of the fast outflow. Additionally, by studying the mass accretion rates and jet outflow momenta, we can determine how the changing conditions in the inflow and outflow affect the launching of these jets and their environment. These results are crucial to model binary post-AGB binaries and PNe. This is important to get a better understanding of the binary interaction physics, such that we can provide a more complete description of the further evolution of these systems and their impact on the surrounding environment.


Spectroscopic variability of proto-planetary nebulae

Van de Steene, Griet

We are carrying out a long-term radial velocity study of seven bright proto-planetary nebulae (PPNe) that possess bipolar or ellipsoidal nebulae and have been classified as “shell” sources, due to the infrared excess caused by their circumstellar shell. These objects are distinct from “disc” post-AGB sources studied by Van Winckel an collaborators. The most popular explanation of the shaping mechanism of bipolar nebulae attributes it to the interaction of a binary companion. However, none of the seven show clear evidence of variability due to binarity ! This puts severe constraints on possible undetected companions. All PPNe are found to vary in velocity, but these are dominated by semi-regular pulsations.We present our latest findings in the ongoing analysis of the line and radial velocity variablity in these objects.


Dust-production of AGB stars in the Solar Neighborhood

Trejo, Alfonso

Asymptotic Giant Branch (AGB) stars dominate the total dust injection into the interstellar medium (ISM) of galaxies. Studies providing total dust injection rates in the Milky Way (Jura & Kleinmann 1989) and nearby galaxies (Riebel et al. 2012; Srinivasan et al, 2016) show the importance of accurately estimating this contribution.In this work we revisit the total dust mass-loss rate from AGB stars in the Solar neighborhood. Such an update is necessary, especially for an all-sky sample, as contrary to recent and old studies.One of the challenges for Galactic and dusty AGB sources is the distance determination, which we are primarily interested in, as they are the highest mass-loss rate objects. Using present-day all-sky infrared facilities (WISE, 2MASS, AKARI). We constructed spectral energy distributions for all the AGB candidates withinwithin 2 kpc from the Sun, which we fit with models from the GRAMS grid (Sargent, Srinivasan & Meixner 2011; Srinivasan, Sargent & Meixner 2011) to estimate their dust-production rates.We find an integrated dust production rate of ~ 4 X 10^-5 Msun/year or an average of ~ 2 X 10^-8 Msun/year per object is obtained. We compare our results to those of the Magellanic Clouds and other Local Group galaxies, for which the distance determination problems do not exist. Separating the contribution into C- and O-rich AGB is alsopresented and is compared with estimates from the LMC and SMC as well.This work presents new insights into the contribution of low- and intermediate-mass stars to the ISM, and the discrepancy between the dust produced by AGB stars and the estimated reservoir in the ISM.


APEX spectral scan of the M-type AGB star R Dor

De Beck, Elvire

Our current insights into the circumstellar chemistry of AGB stars are largely based on studies of carbon-rich stars and stars with high mass-loss rates. In order to expand the current molecular inventory of evolved stars we carried out a spectral scan  of the nearby, oxygen-rich star R Dor, a star with a low mass-loss rate (~10-7 Msun/yr), using the APEX telescope. The spectrum of R Dor is dominated by emission lines of SO2 and the different isotopologues of SiO. We also detect CO, H2O, HCN, CN, PO, PN, SO, and tentatively TiO2, AlO, and NaCl. Sixteen out of approximately 320 spectral features remain unidentified. Among these is a strong but previously unknown maser at 354.2 GHz, which we suggest could pertain to H2SiO, silanone. With the exception of one, none of these unidentified lines are found in a similarly sensitive survey, performed with the IRAM 30m telescope,  of IK Tau an oxygen-rich AGB star with a high mass-loss rate (~5x10 -6 Msun/yr).Radiative transfer models for five isotopologues of SiO (28SiO, 29SiO, 30SiO, Si17O, Si18O) provide constraints on their fractional abundance and radial extent. We derive isotopic ratios for C, O, Si, and S and estimate that, based on our results for 17O/18O, R Dor likely had an initial mass in the range 1.3-1.6 Msun, in agreement with earlier findings based on models of H2O line emission. From the presence of spectral features recurring in many of the measured thermal and maser emission lines we tentatively identify up to five kinematical components in the outflow of R Dor, indicating deviations from a smooth, spherical wind.


In search of the fundamentals of dust formation around evolved stars with the Origins Space Telescope

De Beck, Elvire

The origins Space Telescope (OST) is a mission concept for a new far-infrared space observatory developed through a community-led study sponsored by NASA in preparation for the 2020 Astronomy and Astrophysics Decadal Survey. I will introduce the OST concept, thereby focusing on two science cases related to dust around evolved stars that we developed for the proposed instruments. The first case proposes high-spectral resolution heterodyne observations of the hot, highly excited gas close to the star in order to trace the depletion of different gas-phase species and constrain the fundamental dust-condensation and growth processes. The second case proposes lower-spectral-resolution, but higher sensitivity observations of the CO ladders towards evolved stars in a number of nearby galaxies, providing unprecedented estimates of gas-mass-loss rates in these populations. Combined with sensitive observations of the dust content, this will provide the most reliable dust-to-gas ratios to date, constraining the dust-formation efficiency at different metallicities.


WD+AGB star as the progenitors of type Ia supernovae

Wang, Bo

The origin of the progenitors of type Ia supernovae (SNe Ia) is still uncertain. The core-degenerate (CD) channel has been proposed as an alternative way for the production of SNe Ia. In this channel, SNe Ia are formed at the final stage of common-envelope evolution from a merger of a carbon-oxygen white dwarf (CO WD) with the CO core of an asymptotic giant branch companion. However, the birthrates of SNe Ia from this scenario are still not well determined. In this work, we performed a detailed investigation on the CD channel based on a binary population synthesis approach. The SN Ia delay times from this scenario are basically in the range of 90Myr-2500Myr, mainly contributing to the observed SNe Ia with short and intermediate delay times although this scenario can also produce some old SNe Ia. Meanwhile, our work indicates that the Galactic birthrates of SNe Ia from this scenario are no more than 20% of total SNe Ia due to more careful treatment of mass transfer. Although the SN Ia birthrates in the present work are lower than those in Ilkov & Soker, the CD scenario cannot be ruled out as a viable mechanism for the formation of SNe Ia. Especially, SNe Ia with circumstellar material from this scenario contribute to 0.7-10% of total SNe Ia, which means that the CD scenario can reproduce the observed birthrates of SNe Ia like PTF 11kx. We also found that SNe Ia happen systemically earlier for a high value of metallicity and their birthrates increase with metallicity.


Populations of accreting white dwarfs

Chen, Hai-Liang

Accreting white dwarfs are important for studies of SNe Ia, X-ray and UV emission of elliptical galaxies and nova population. We have modelled the formation and evolution of populations of accreting white dwarfs in different types of galaxies. I will show the results of X-ray and UV emission from accreting white dwarfs in elliptical galaxies and comparison of our results with observations of Chandra, SDSS . Moreover, I will talk about the properties of nova population in different types of galaxies.


Near-infrared polarimetry of M2-9

Navarro, Silvana G.

We present the near-infrared polarization observations of M2-9, a highly bipolar planetary nebulae with a binary central object. The polarization observations were obtained with the near-infrared polarimeter POLICAN at the Guillermo Haro Observatory.We compare them with the optical polarization obtained at the San Pedro Martir Observatory using the polarimeter POLIMA.


ALMA spectrum of the extreme OH/IR star OH26.5+0.6

Justtanont, Kay

We present ALMA band 7 data of OH 26.5+0.6 in band 7, resolving the CO J=3-2 emission which shows a compact core (FWHM = 0.6 arcsec), surrounded by tenuous envelope which extends out to 4-5 arcsec where CO is photodissociated. The circumstellar envelope also exhibit  a number of emission lines due to metals, e.g., NaCl and KCl. A lack of C$^{18}$O is expected but a nondetection of C$^{17}$O is puzzling given the strength of H$_{2}$^{17}$O from the Herschel spectrum of the star. However, a line associated with Si$^{17}$O is seen which may be indicative in molecular formation processesin envelope of extreme OH/IR stars. We also report a detection of the gas-phase emission line of MgS J=21-20. MgS dust is found exclusively in C-rich circumstellar environment and is thought to be the carrier of the 30$\mu$m dust feature. ALMA spectrum of this object reveals intriguing features which may be used to investigate chemical processes and dust formation during a high mass-loss phase.


Models of binaries containing giants

Yakut, Kadri

In this study, we modelled 60 relatively long-period binaries with giant component(s) using their observed physical parameters. Most of the selected systems consisting of a cool, evolved star and a hotter less evolved component. We compared our model results with the observed mass, radius and temperature values for each components in the systems. Theoretical models include core convective overshooting, mass-loss, and tidal friction. We found out that some of the systems have overevolved secondary. Explanation being that the primary component is the merged remnant of a former short-period sub-binary in a former triple system. We will give the detail of study in this presentation.


Mass-loss from solar-like and metal-poor stars

McDonald, Iain

Mass-loss prescriptions for solar-mass stars overpredict the number of AGB stars: we have been taking new observations to better calibrate this relationship and understand the processes of mass loss. I will report on our progress into: (1) new evidence showing RGB mass loss is very different to current predictions, (2) the onset of pulsation-driven mass loss once stars reach a pulsation period of ~60 days, (3) the similarities in dust production across a factor of at least 40 in metallicity. I will close by mentioning how this research ties into the NESS and DUSTiNGS surveys and give direction for a new universal mass-loss prescription.


The maser emitting structure and time variability of the SiS lines J=14-13 and 15-14 in IRC+10216

Fonfria, Jose Pablo

AGB stars are important contributors of processed matter to the ISM. However, the physical and chemical mechanisms involved in its ejection are still poorly known. This process is expected to have remarkable effects in the innermost envelope, where the dust grains are formed, the gas is accelerated, the chemistry is active, and the radiative excitation becomes important. A good tracer of this region in C-rich stars is SiS, an abundant refractory molecule that can display maser lines, very sensitive to changes in the physical conditions. We present high angular resolution interferometer observations (HPBW>0.25") of the v=0 J=14-13 and 15-14 SiS maser lines towards the archetypal AGB star IRC+10216, carried out with CARMA and ALMA to explore the inner 1" region around the central star.The observations reveal a complex emitting structure with three major components: (1) two blue- and red-shifted strong spots, (2) an elongated component around the systemic velocity, and (3) a weaker extended emission. We have modeled the observations in detail with a 3D radiation transfer code finding that they cannot be reproduced only by thermal emission. About 75% of the total emission is modeled by several maser spots with different velocities and a thin shell-like maser structure, necessary to reproduce the extended emission. The remaining 25% is thermal emission.These observations have been complemented with a spectral monitoring of several SiS lines carried out with the IRAM 30m telescope every 14 days from 2015 to present. The intensity of some spectral maser components strongly depend on the stellar pulsation while other maser components show a mild variability. We also discovered a significant phase lag of ~0.2 between the maser and the NIR light-curves.The innermost envelope of IRC+10216 is a quite complex region substantially affected by the evolution of the central star. This and future works about this region will shed light on the matter ejection problem.


Long Period Variables in dwarf Irregular Galaxies of the Local Group

Menzies, John

In a survey of Local Group galaxies conducted at SAAO, we have included a number of dwarf irregular galaxies. We discovered some remarkable long period variables in the galaxies, NGC 6822, SagDIG and IC1613, and we now have identified more than 10 Miras in WLM and at least 7 in NGC 3109. We discuss the distribution of periods and the implications for theoretical modelling of these stars in the late stages of AGB evolution.


Carbon stars in the Magellanic Clouds: properties and dust production rate

Nanni, Ambra

We employ newly computed grids of spectra reprocessed by dust to fit the spectral energy distributions (SEDs) of the entire sample of carbon-stars (C-stars) in the Small Magellanic Cloud (SMC). This procedure allows us to derive some important properties of these stars as well as their dust production rate (DPR). For the first time, the grids are calculated as a function of the stellar parameters, i.e. mass-loss rate, luminosity, effective temperature, current mass and carbon-excess, following a consistent, physically grounded scheme of dust growth coupled with stationary wind outflow. Our model accounts for the dust growth of various dust species formed in the circumstellar envelopes of C-stars, such as carbon dust, silicon carbide and metallic iron. The available grids are computed for different combinations of optical constants and grain sizes for carbon dust that have been shown to simultaneously reproduce the most relevant infrared colour-colour diagrams in the SMC.Differently from the other works in the literature, our approach allows for the direct estimate of the mass-loss and of the DPR of these stars, without the need of assuming the gas-to-dust ratio, the outflow expansion velocity and the dust chemistry. These latter quantities are indeed consistently calculated by in our dust growth scheme.The DPR provided by our method can be significantly different, between a factor of 2-5, from the ones available in the literature.The same kind of investigation is currently ongoing for C-stars in the Large Magellanic Cloud and we here show our preliminary results.


A step further on the physical, kinematic and excitation properties of PNe

Gonçalves, Denise R.

Planetary nebulae (PNe) are known to be photoionized objects. However they also have small-scale, low-ionization structures (LIS) with different behaviour in terms of excitation. We are only now starting to answer why most LIS have lower electron densities than the PN shells hosting them, and whether or not their intense emission in low-ionization lines is the key to their main excitation mechanism. Therefore, can LIS line ratios, chemical abundances and kinematics enlight the interplay between the different excitation and formation processes in PNe? Based on the spectroscopic analysis of five PNe that possess LIS, we  confirm that all nebular components have comparable electron temperatures, whereas the electron density is systematically lower in LIS than in the surrounding nebula. We also confirm that there is no chemical abundance contrast between LIS and other PN components. More importantly, by using diagnostic diagrams from shock models, we demonstrate that LIS's main excitation is due to shocks, whereas the other components are mainly photoionized. We propose new diagnostic diagrams involving a few emission lines ([N II], [O III], [S II]) and log(fshocks/f*), where fshocks and f* are the ionization photon fluxes due to the shocks and to the central star ionizing continuum, respectively. The new diagrams clearly separate the structures of the PNe in photo- and shock-excited components, also defining a region where both mechanisms play a role.


The Impact of Dust/Gas Ratios on Chromospheric Activity in Red Giant and Supergiant Stars

Carpenter, Ken

Stencel et al. (1986) analyzed IUE spectra of a modest sample of cool stars and found that such objects continue to produce chromospheres even in the presence of high dust levels in the outer part of their atmospheres. This reversed the previous results of Jennings (1973) and Jennings & Dyck (1972). We present here an extension of the Stencel et al. study to a larger sample of stars representing a broader range in dust/gas ratios, using archival IUE and HST data on both RGB and AGB stars. Surface fluxes in emission lines will be used to assess the chromospheric activity and obscuration by dust in each star, as those fluxes will follow a different pattern for reduced activity (temperature/density dependent) vs. dust obscuration (wavelength dependent). The goal is to assess the applicability of the Stencel et al. conclusions in a larger and more diverse sample of stars.


An episodic maser jet in the water fountain IRAS 18113-2503

Orosz, Gabor

Planetary nebulae often express beautiful, multipolar, point symmetric shapes - while their progenitors are characterised as having spherical layered structures. As stars exhaust their nuclear fuel and depart from the AGB, they launch powerful jets that are believed to be responsible for creating the diverse shapes of PNe. Spectacular examples of collimated jets are those traced by water masers, in objects named "water fountains" (with jet speeds and sizes on the order of 100 km/s and 1000 AU respectively). Water fountains are very rare due to their short timescales (on the order of 100 years) and as such the launching mechanism of their collimated jets remains an unresolved puzzle. In our talk, we introduce the VLBI maser astrometric results of IRAS 18113-2503, a water fountain with multiple bipolar bow shocks in its high-velocity collimated outflow. Using our VLBA and VERA measurements, we unveil the spatial/kinematic structure and trigonometric distance (>10 kpc) of the bipolar bow shocks. We argue that the measured 3D maser velocities (>200 km/s) clearly show that the jets are formed in very short-lived, episodic outbursts on a decadal timescale, and that they decelerate in a non-linear way. Using a simple kinematic model, we use our astrometric results to derive the physical properties of the jet, its source and surroundings, and argue that the most reasonable driving mechanism is a binary system.


Galactic Simulations of r-process Elemental Abundances

Haynes, Chris

We present the results of chemodynamical simulations with the inclusion of four different neutron capture processes: magneto-rotational supernovae, neutron star mergers, neutrino driven winds and electron capture supernovae. We examine both simple isolated gas discs and cosmological zoom-in simulations of a Milky way-type galaxy and compare the [Eu/Fe] and [Eu/O] evolution with recent observations, including the HERMES-GALAH survey. We find that neither electron capture supernovae or neutrino driven winds are able to adequately produce neutron capture elements to match observations. Both neutron star mergers and magneto-rotational supernovae are able to produce the elements in sufficient quantities; additionally we find that the scatter in [Eu/Fe] at low metallicity ([Fe/H] < -1) and the [Eu/O] slope at high metallicity ([Fe/H] > -1) give an important constraint on the r-process sites. Using these we predict that neither neutron star mergers or magneto-rotational supernovae are able to explain the observed europium abundances separately and can better match observations with a significant contribution from both.


Population of AGB stars in the Outer Galaxy

Szczerba, Ryszard

Working on a systematic study of star formation in the Outer Galaxy, we have found that hydrodynamical models of AGB and post-AGB evolution (Steffen, Szczerba, Schoenberner, 1998 A&A, 337, 149) allow for identification of AGB stars of both chemistry on some color-color diagrams. In this respect, K-[24] vs. K-[8] diagram is the best for sources detected in [24] MIPS band (Szczerba et al. 2016,  J. Phys.: Conf. Ser. 728, 042004). During my talk I will present details of the selection method, and will discuss properties of AGB stars selected from the “Spitzer Mapping of the Outer Galaxy” survey (SMOG; PI Sean Carey) that covered ~24 deg2 region in the Outer Galaxy: l = (102o, 109 o), b = (-0.2 o, 3.2 o) in the IRAC 3.6–8.0 µm and MIPS 24 µm bands. The analyzed sample contains about 3 millions sources, however, mostly with purely photospheric colors.  The selected population of AGB stars from the Outer Galaxy will be compared to the populations of such stars in Magellanic Clouds, as well as to the sample of AGB stars identified toward Galactic bulge. 


Photometric properties for a carbon star DARWIN model grid

Eriksson, Kjell

We have computed a new grid of Radiation HydroDynamic models for carbon-rich AGB-stars with the DARWIN code, this time using size-dependent dust opacities (instead of using the small particle approximation) but otherwise like the grid in Eriksson et al. 2014 (A&A 566, A95). The main results of this modification are that the mass-loss rate vs outflow velocity relation is rather unaffected and that the dust condensation degree is roughly halved.The photometric properties of the grid, as computed by the COMA code, will be presented. Specifically, synthetic magnitudes in the Johnson-Cousins BVRI and the Johnson-Glass JHKLL'M systems were calculated.


Where is the metallicity ceiling to form carbon stars?

Boyer, Martha

Carbon stars form easily at low metallicity due to efficient dredge up and a low surface abundance of oxygen that leads  o C/O>1 in their atmospheres. At the AGB III meeting in Vienna, we presented a pilot program that targeted the inner disk of M31 and hinted at a metallicity ceiling above which carbon stars cannot easily form. Here, we present new HST observations in M31 that revisit the question of a metallicity ceiling for carbon stars. We targeted 20 fields across the disk, taking advantage of M31's metallicity gradient and diversity in stellar age. The results show a clear effect at the highest metallicity, with a drop in the C/M ratio of more than an order of magnitude from [Fe/H] = -0.2 to [Fe/H] = 0. These observations make use of the medium band filters on WFC3/IR, which are a powerful and efficient tool for identifying carbon stars in distant galaxies by sampling key near-infrared molecular features. To demonstrate the potential of this technique, I will also briefly discuss results from another program that uses the same observing strategy in nearby dwarf galaxies to identify dust-producing AGB stars. Finally, I will discuss how to do similar observations with JWST.


The interaction of the Helix nebula with the interstellar medium; a solid case.

Manchado, Arturo

As an asymptotic giant branch (AGB) star loses mass, moving with respect to the interstellar medium (ISM),its morphological shape is affected and a characteristic bow shock appears. This is the case for the planetary nebula (PN) NGC 7293 (The Helix nebula).This PN represents one of the rare instances in which theoretical predictions of stellar evolution can be accurately tested against observations.The distance is well determine, thus the mass of the progenitor star that has evolved into a PN, and the velocity and proper motion of the central star is known.We performed numerical simulations of the interaction of the Helix nebula with the local ISM and constrain  the evolution of both the progenitor and the nebula.The simulations reproduce well the observations, in particular, the multiple bow-shock structures ahead of the star can be explained by multiple fragmentation ofthe shock front where the direct interaction of the stellar wind with the ISM takes place.The outer nebular size (or PN halo) is the result of the mass-loss associated to the last thermal pulse only, i.e. the last 50 000 yr of evolution on the AGB phaseand the PN-ISM interaction.


Dust Structure around Asymptotic Giant Branch Stars

Upadhyay, Devendra Raj

This paper presents mass, temperature profile variation of Planck function in different region, energy out flow nature. The physics of the Interstellar medium (ISM) is extremely complex because the medium is very inhomogeneous and is made of regions with fairly diverse physical conditions. The composition of our galaxy is made of stars that provide a mass of approximately 1010-1011 solar masse and the ISM that provides a mass of nearly 109 solar mass. The conversion of the ISM to stars occurs typically at the star formation rate of 3 solar mass per year. The contribution of mass from the stars to the ISM occurs through supernovas (0.3 solar mass per year), staller winds of OB stars (0.08 -0.5 solar mass per year) and stellar winds from red giants (0.3 -1 solar mass per year). Approximately 99% of the mass of the interstellar medium is in the form of gas with the remainder primarily in dust. The asymptotic giant branch (AGB) is the region of the Hertzsprung-Russell diagram populated by evolving low to medium mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars (0.6–10 solar masses) late in their lives. Asymptotic giant branch (AGB) stars are generally classified as oxygen-rich (M-type) or carbon-rich (C-type) based on the chemistry of the photosphere and or the outer envelope.The physical phenomena such as dust, grain encountered in arena of ISM are inhomogeneous and anisotropy, the technique to solve such problem will be described.


Dust producing AGB stars in the Metal-poor, Dwarf Irregular Galaxies Sextans A and Leo A

Jones, Olivia

We present JHK s observations of the metal-poor ([Fe/H] < -1.40) dwarf-irregular galaxies, Leo A and Sextans A, obtained with the WIYN High-resolution Infrared Camera at Kitt Peak. Their near-IR stellar populations are characterized by using a combination of color–magnitude diagrams and by identifying long-period variable stars. We detected red giant and asymptotic giant branch stars, consistent with membership of the galaxy’s intermediate-age populations (2–8 Gyr old). Matching our data to broadband optical and mid-IR photometry, we determine luminosities, temperatures, and dust-production rates (DPR) for each star. We identify 32 stars in Leo A and 101 stars in Sextans A, confirming that metal-poor stars can form substantial amounts of dust. We also find tentative evidence for oxygen-rich dust formation at low metallicity, contradicting previous models that suggest oxygen-rich dust production is inhibited in metal-poor environments.  The majority of this dust is produced by a few very dusty evolved stars and does not vary strongly with metallicity


Updates on the Ultraviolet Emission from Asymptotic Giant Branch Stars

Montez, Rodolfo

A comprehensive study of the UV emission detected from asymptotic giant branch (AGB) stars by the Galaxy Evolution Explorer (GALEX) revealed that of the 316 AGB stars observed by GALEX, 57% were detected in the near-UV (NUV) bandpass and 12% were detected in the far-UV (FUV) bandpass. The NUV emission from AGB stars is correlated with the optical to the near-infrared emission and is often found to vary in phase with phased visible light curves. The detections and non-detections indicate higher detection fractions among the brightest, and hence closest, AGB stars, as well as the influence of galactic extinction. The study also found evidence for anti-correlation between the circumstellar envelope density and the NUV – and possibly FUV – emission. Altogether, these results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars, likely traced to a combination of photospheric and chromospheric emission. Here, we extend our initial study (Montez et al. 2017) to include additional archival spectroscopic UV observations as well as an expanded sample of UV-variable AGB stars, in an effort to better understand the nature of AGB star UV emission.


The interplay between pulsation, mass loss, and third dredge-up: More about Miras with and without technetium

Uttenthaler, Stefan

We follow up on a previous paper (Uttenthaler, 2013) that showed that Miras containing the 3DUP-indicator technetium (Tc) in their atmosphere follow a different sequence of dust mass-loss rate as a function of pulsation period than Miras without Tc, where the dust mass-loss rate is probed by a near- to mid-infrared colour such as K-[22]. Contrary to what one might naively expect, Tc-poor Miras show redder K-[22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at similar periods. In the meantime, the previous sample was extended and the analysis was expanded towards other colours and dust spectra. Near- and mid-IR photometry and ISO dust spectra of our stars were investigated where available. We analysed new optical spectra and expanded the sample by including more stars from the literature. We also investigated if the same two sequences can be revealed in the gas mass-loss rate. For this end, literature data of gas mass-loss rates of Miras and semi-regular variables were collected and analysed. Our results show that Tc-poor Miras are redder than Tc-rich Miras in broad range of the mid-IR, suggesting that the previous finding based on the K-[22] colour is not due to a specific dust feature. Probably, Miras without Tc experience a higher mass-loss rate than Miras with Tc. The relation log(M_dot,gas)=0.393x(K-[22])-7.431 is found to be valid in a broad range of K-[22] colour. We also find that the 13 µm feature disappears above K-[22]~=2.17, corresponding to M_dot,gas~=2.6x10-7 Msun/yr. Similar sequences of Tc-poor and Tc-rich Miras in the gas mass-loss rate vs. period diagram were however not found, most probably owing to limitations in the available data. Different hypotheses to explain the observation of two sequences in the P vs. K-[22] diagram are discussed and tested. We conclude that the interplay between pulsation, mass loss, and 3DUP deserves further attention.


Elemental abundances of primary stars in Sirius-like systems: Constraints on pollution from AGB stars

Bharat Kumar, Yerra

Study of primary stars lying in Sirius-like systems with various masses of WD companions and orbital separations is one of the key aspects to understand the origin and nature of Barium (Ba) stars, wherein enhanced carbon and/or s-process elements are suggested through mass transfer from AGB companions. We have analysed chemical composition of 21 FGK primaries, including dwarfs and giants, systematically using good quality high resolution spectra. Three Ba dwarfs are discovered which provides the first direct evidence of Ba dwarfs are with WD companions. On the other hand, absence of s-process enrichment is observed in stars even their companion WD mass is large enough to produce these elements during their TP AGB phase. Surface abundances of s-process elements in our sample are compared with AGB models of respective masses estimated from their WD masses. Results suggest s-process enrichment in sample Ba dwarfs follow McClure hypothesis similar to Ba giants. A few conclusions are drawn from this study: large mass of WD (>0.51 Msun) is not only the sufficient condition to form a Ba star, orbital separation is not main constraint to differentiate between strong and mild Ba stars, and different levels of s-process abundances among Ba stars may not be dominated mainly by the metallicity.


Infrared Studies of the Variability and Mass Loss of Some of the Dustiest Asymptotic Giant Branch Stars in the Magellanic Clouds

Sargent, Benjamin

The asymptotic giant branch (AGB) stars with the reddest colors have the largest amounts of circumstellar dust, which in turn suggests extremely high mass-loss rates.  AGB stars vary in their brightness, and studies show that the reddest AGB stars tend to have longer periods than other AGB stars and are more likely to be fundamental mode pulsators than other AGB stars.  Such dusty AGB stars are difficult to study, as their colors are so red due to their copious amounts of circumstellar dust that they are often not detected at optical wavelengths.  Therefore, they must be observed at infrared wavelengths to explore their variability.  Using the Spitzer Space Telescope, my team and I have observed a sample of very dusty AGB stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) over Cycles 9 through 12 during the Warm Spitzer mission.  For each cycle, we typically observed a set of AGB stars at both 3.6 and 4.5 microns wavelength approximately monthly for most of a year.  These observations reveal a wide range of variability properties.  I present results from our analysis of the data obtained from these Spitzer variability programs, including light curve analyses and comparison to period-luminosity diagrams.  Potentially the most interesting set of stars we observed is the sample of 13 stars from the LMC included in the set of so-called Extremely Red Objects (EROs) that were studied by Gruendl et al (2008, ApJ, 688, L9), who determined these carbon stars to have among the highest mass-loss rates of the LMC's carbon-rich AGB star population.  Paradoxically, we found most of these stars to have little to no variability, perhaps suggesting these stars are reaching or have just reached the end of the AGB phase of their lives.


To Be or Not to Be: EHB Stars and AGB Stars

Brown, David

The formation and lives of EHB stars are linked to the lives of AGB stars by indications that such EHB/sdB stars might form in globular clusters with multiple populations (D’Antona et al., 2005) linked to AGB evolution.  Various scenarios have been proposed by which EHBs may form. Binary hypotheses (Mengel, Norris & Gross 1976) posit that they form from the interaction of stars in a binary system via mass transfer, leaving either an EHB binary or a single EHB star, the result of a merger between two He WD stars (Han et al., 2002, 2003). Single star formation scenarios invoke either high stellar winds during the RGB phase of evolution (D’Cruz et al. 1996) before the He-flash, or a delayed He-flash after rapid mass loss (Sweigart 1997),  or even mass loss during the HB phase itself (Yong et al. 2000). More recently, observations of multiple subpopulations in massive globular clusters (Piotto et al., 2005), such as ?-Centauri (Bedin et al., 2004), along with the detection of a Na-O anti-correlation (Carretta et al., 2006), suggest that single EHB stars might form from He-enhanced progenitors (D’Antona et al., 2005, 2008; Lee et al., 2005) in environments enriched by AGB ejecta. This scenario is one to be considered given the disparity in EHB/sdB binary fraction between the Galactic field (f~2/3; Maxted et al., 2001) and in globular clusters (f?0.04-0.14; Moni Bidin et al., 2011). The studies conducted by Han et al. (2002, 2003, 2007) have been able to provide a strong case for the binary formation of EHB/sdB stars in the Galactic field, but more work must be done to investigate how EHB stars, possibly in binaries, might be able to form in globular clusters. The results of simulations presented here are an extension of the simulations of Han et al. (2002, 2003), done for Z=0.02, to a diverse range of metallicities in order to see how binary channels in chemically diverse environments might contribute to the overall EHB population in globular clusters.


A THEORY AND SIMULATION-BASED CONVECTIVE BOUNDARY MIXING MODEL FOR AGB STAR EVOLUTION AND NUCLEOSYNTHESIS

Battino, Umberto

The s-process nucleosynthesis in Asymptotic Giant Branch (AGB) stars depends on the modelling of convective boundaries. I present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries, where Hydrodynamics simulations suggest the presence of Convective Boundary Mixing (CBM). In this work I apply a CBM model purely motivated by simulations and theory, with no finetuning by hand "a priori" to match observations, to 1D stellar evolution models with initial mass M = 2 and M = 3 solar masses , and with initial metal content around the solar value. This mixing is affecting the Ne22(gamma,n)Mg25 activation and the s-process efficiency in the C13-pocket. The theoretical results are in good agreement with observations: at the end of the AGB evolution, we obtain an s-process production 0.36<[s/Fe]<0.78 and -0.23<[hs/ls]<0.45, which is consistent with spectroscopic observations of C-rich AGB stars.


Mass Loss Rates of Li-rich AGB/RGB Stars

Maciel, Walter

A sample of AGB/RGB stars with an excess of Li abundances is considered in order to estimate their mass loss rates. Our method is based on a correlation between the Li abundances and the stellar luminosity, using a modified version of Reimers formula. We have adopted a calibration on the basis of an empirical correlation between the mass loss rate and some stellar parameters. We conclude that most Li-rich stars have lower mass loss rates compared with the majority of AGB/RGB stars, which show no evidences of Li enhancements, so that the Li enrichment process is apparently not associated with an increased mass loss rate.


The star formation history in the M31 galaxy derived from Long-Period-Variable star counts

Torki, Maryam

The determination of the star formation history is a key goal for understanding of galaxies. In this paper we developed a new method to reconstruct the star formation history in the Andromeda galaxy. This galaxy is a nearest and biggest spiral galaxy to the milky way and this is a great chance for us to study the formation and evolution of this galaxy. Over findings suggers that long period variables stars are the powerful tool for estimate the star formation history in nearby galaxies because of their evolutionary phase. In this method at first we convert-k band magnitude of evolved stars to mass and age and from this we reconstruct the star formation.


A new Python interface for the spectroscopic data analysis.

El jariri, Youssef

Recently we have carried out a campaign of measurements of spectra of variablestars at the Oukaimeden observatory. To analyse thousands of spectra, we developed agraphical interface under Python. Spectroscopic study of variable stars is based on theacquisition of data in the observatory, and their storage in fit files. We treat thisdata digitally to make them exploitable and graphically representable, for this purposewe use the special software or scripts developed by the astrophysicists. To facilitateanalysis and meet our needs, we developed a graphical interface using Python, we usingthe Tkinter module to represent graphically our spectroscopic data. This interfaceisable toplot figures of our spectra on 1D, and as a superposition on 1D and 2D. Thus, thisinterface can determine the necessary parameters in the spectroscopic study of short-period variable stars; such as the pulsation phase, the Blazhko phase, the correction ofthe heliocentric velocity... Moreover, we can use this interface to convert the pre-treatedfit files into text filesas well as to create Excel tables containing the calculatedparameters. This interface is easy, fast to use, andcan provide high quality figures.


AGB stars of the Magellanic Clouds as seen within the Delta-a photometric system

Paunzen, Ernst

About four decades ago, the Delta-a photometric system was introducedin order to investigate the flux depression at 5200A, typically for chemicallypeculiar stars of the upper main sequence. The a-index samples the flux of the5200A region by comparing the flux at the center with the adjacent regions.The final intrinsic peculiarity index Delta-a was defined as the difference between theindividual a-values and the a-values of normal stars of the same colour (spectral type).Here we present, for the first time, a case study to detect and analyse AGBstars in the Magellanic Clouds. For this, we use synthetic spectra and ourphotometric survey of the Magellanic clouds within the a-index. We find thatAGB stars can be easily detected on the basis of their Delta-a index in anefficient way. We discuss our findings for AGB-stars of different chemistries. 


High angular-resolution infrared imaging and spectra of the carbon-rich AGB star V Hya

Sahai, Raghvendra

The post-AGB star V Hya is believed to be in the very brief transition phase between the AGB and a planetary nebula.  It is likely in the earliest stages of this transition of any known object. Using STIS/HST, we previously found a high-velocity (>200 km/s) jet or blob of gas ejected only a few years ago from near (<0.3'') the star.   From multi-epoch high-resolution spectroscopy we previously found time-variable high-velocity absorption features in the CO 4.6 micron vibration-rotation lines of V Hydra.   Modeling shows that these are produced in compact clumps of outflowing gas with significant temperature gradients.  Here we present very high resolution (~100 mas) imaging of the central region of V Hya using the coronagraphic mode of the Gemini Planet Imager (GPI) in the 1 micron band and spectral-spatial imaging of 4.6 micron CO 1-0 transitions using the Phoenix spectrometer.   We report successful detection of a compact central dust disk from GPI and molecular emission from the Phoenix observations at relatively larger scales. We discuss models for the central structures in V Hya, in particular disks and outflows, using these and complementary images in the optical and radio.


The loss of large amplitude pulsations at the end of AGB evolution

Engels, Dieter

OH/IR stars are thought to belong to the more massive part of the AGB and early post-AGB population, and are obscured due to optically thick dusty envelopes. The envelopes harbour luminous 1612 MHz OH maser emission, which can be used to monitor the stellar variability. The stars are either large amplitude variables with periods up to 7 years, or show small or even no amplitude variations. This dichotomy in the variability behaviour is assumed to mark the border between the AGB and the post-AGB evolutionary stages. As both groups are apparently disjunct, the cessation of large amplitude pulsation must occur over a very short period (<<1000 years). A current monitor program (https://www.hs.uni-hamburg.de/nrt-monitoring) of a sample of several dozen OH/IR stars made with the Nancay Radio Telescope at 1612 MHz, tries to find objects in transition and to describe their variability properties. We consider the fading out of pulsations with steadily declining amplitudes as a viable process. Promising candidates are presented in this Poster.


Hen 3-160 - the first symbiotic binary with Mira variable S star?

Galan, Cezary

Hen 3-160 is reported in Belczynski et al.’s (2000) catalogue as symbiotic binary system with M7 giant donor. Using V and I-band photometry collected over 14 years we have found from that the giant is a Mira variable pulsating with 242.5-day period. The period-luminosity relation locates Hen 3-160 at the distance of about 9.5 kpc, and its Galactic coordinates (l=267.7, b=-7.9) place it ~1300 pc above the disc. This position combined with relatively high proper motions (pmRA=-1.5 mas/yr, pmDEC=+2.9 mas/yr; Gaia DR2) indicates that Hen 3-160 has to be a Galactic halo object. Our red optical and infrared spectra show the presence of ZrO and YO molecular bands that appear relatively strong compared to the TiO bands. Here we propose that the giant in this system is intrinsic S star, enriched in products of slow neutron capture processes occurring in its interior during AGB phase which would make Hen 3-160 the first symbiotic star with Mira variable S star.


Kepler K2: A Search for Very Red Stellar Objects

Hartig, Erich

Continuing the analysis of Long Period Variables (LPVs) observed as part of the Kepler K2 mission we show the latest results. We found that the 'Kepler K2 High Level Science Product' (K2HLSP) 'Self-Flat-Field' method (K2SFF or SFF) corrected the instrumental effects best. An iterative fitting method using sinusoidal functions was developed and evaluated.Furthermore, we present a strategy to properly constrain periods longer than the observational time basis using rms-error minimization. To check the results of this method, we applied it to a 33 d long dataset (1560 data points) of EP 202070273 (Kepler K2 Campaign 0) for which a ground based period of 730 d is available. The output from our method, 763 d, is in excellent agreement.From the Kepler K2 Campaign 2 we analysed a total of 41 targets data that are suspected or known LPVs. The 77.48 d of observations provided roughly 3500 data points. We searched for the SFF sinusoidal fit at the global rms-minimum and compared the periods with the available ASAS periods. In this way we defined the error range which could be applied to the rest of the targets.The preliminary results are presented in the poster.


Raman and Thomson Wings around Balmer Lines in the Two S-Type Symbiotic Stars Z Andromedae and AG Draconis

Chang, Seok-Jun

Symbiotic stars are binary systems composed of a hot white dwarf and a mass losing giant. Broad wings around Balmer emission lines are found in many symbiotic stars including the two S-type symbiotic stars, Z Andromedae and AG Draconis. Monte-Carlo simulations are carried out to produce Balmer wings from Thomson scattering with free electrons and Raman scattering with atomic hydrogen. Thomson wings of Ha and Hß have the same widths in the Doppler space due to the cross section independent of wavelength. In contrast, Raman Ha wings are 3 times broader widths than Hß counterparts, which is attributed to the different cross sections and branching ratios. The half width at half maximum (HWHM) is proportional to Te1/2 for Thomson wings and NHI1/2 for Raman scattering. We present our spectra of Z And and AG Dra obtained with the Canada-France-Hawaii Telescope, where Ha wings are found to be broader than Hß wings indicative of significant contribution of Raman scattering to the formation of the Balmer wings.


Stellar Wind Accretion and Raman O VI Spectroscopy of the Symbiotic Star AG Draconis

Lee, Young-Min

Symbiotic stars(SySts) are wide binary systems of a mass losing giant and a hot white dwarf. Nearly half of SySts exhibit broad Raman scattered O VI scattered features at 6825Å and 7082Å. Formed through inelastic scattering of O VI??1032 and 1038 with neutral hydrogen, they provide important information about the mass loss and mass transfer processes. Adopting a Monte Carlo method, we perform a profile analysis of Raman O VI features of the yellow SySt AG Draconis based on asymmetric Keplerian accretion flow around the white dwarf. Our best fit to the spectra obtained with the Canada-France-Hawaii Telescope is obtained from our model with a mass-loss rate of the giant ~ 4 × 10-7 M_sun yr-1. A slight red excess in the observed data suggests the presence of the bipolar neutral component receding directions perpendicular to the O VI emission region with a speed ~ 70 km s-1.


K-Type Supergiants in the Large Magellanic Cloud

Wing, Robert

The brightest stars of the Large Magellanic Cloud are supergiants with magnitudes in the V = 11-14 range and a very uneven distribution in color: there are large numbers of both very blue (types O, B, and A) and very red (type M) supergiants, but only a few confirmed supergiants of intermediate types (F, G, K). The lack of K supergiants in the Large Cloud is particularly noteworthy because the Small Magellanic Cloud contains large numbers of them. Are K-type supergiants really absent from the LMC, or have they simply been missed by the survey techniques employed to date? Near-infrared objective-prism surveys by Westerlund and Blanco identified several hundred M supergiants on the basis of their TiO bands but had no way of identifying K stars. On the other hand, the Case survey by Sanduleak and Philip used blue objective-prism plates of such low dispersion that no spectal features (not even TiO) could be seen, but they catalogued several hundred "suspected late-type supergiants" on the basis of their red slope in the blue region and an apparent magnitude in the right range to be supergiant LMC members. Most of the Case sample had not been picked up in the Westerlund or Blanco surveys and must have had types earlier than M2. Does the Case sample include the "missing" K-type supergiants, or are they foreground stars of lower luminosity? To settle this question, the writer has been obtaining two-dimensional spectral classifications for the Case suspected supergiants which were not already known to be M stars - a total of 433 targets. The observations consist of narrow-band photometry on a 5-color system that measures bands of TiO and CN in the near infrared. Although the follow-up observations are not yet complete, nearly 100 stars have been confirmed as K stars of luminosity class I, while many others are M stars of types M0 and M1 which escaped detection on the TiO-based infrared surveys. Only a small minority are found to be foreground stars of lower luminosity.


We determine the distance 91 longer period Mira variables using 3.4µm Period Luminosity Relation

Urago, Riku

We present distance determination 91 for longer (log P > 2.7) period Mira variables (also known as OH/IR stars) covered with thick circumstellar dust shells. They are thought to be younger (~100Myr) population and could be a good tracer of the arm structure of our galaxy. However, they have thick dust circumstellar dust shells and are heavily reddened by circumstellar extinction.In order to estimate their distance using Period-Luminosity Relation (PLR), we need to consider the contribution of circumstellar extinction. In fact, the K band PLR for the Mira variables in LMC shows large scatter for the longer period and so it is difficult to estimate the distance. In contrast, the 3.4 um PLR we constructed using OGLE-III Mira variables in LMC shows a tight and linear correlation with the root mean square residuals of 0.29 mag for the period range of  2.0<logp<3.0. it is necessary to correct the only interstellar extinction when we apply the 3.4um plr for observed data because the 3.4um plr contains the contribution of circumstellar extinction in it. to estimate the amount of interstellar extinction, we need to know the intrinsic color of long period mira variables including circumstellar extinction. we examined the j-h and h-k colors of lpvs founded in the catalina sky survey. since css survey covers only high galactic latitude, we can reasonably assume that they suffer almost no interstellar extinction but includes the contribution of circumstellar extinction, if they have.="" we found that css lpvs show the linear sequence in the j-h and h-ks color diagram, and can estimate the amount of interstellar extinction comparing the position of apparent color with them.finally, we demonstrate the galactic distribution of 91 longer-period mira variables monitored at kagoshima university for which the 3.4um plr is applied and the interstellar extinction is estimated by our new method.<="" p="">
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New PARSEC-V2.0 stellar isochrones with alpha-enhanced mixtures and rotation

Costa, Guglielmo

We present a large set of rotating and not rotating evolutionary tracks for initial masses between0.1 and 350 solar masses. We discuss the method adopted to calibrate the free parameters associated with rotation and convective overshoot.We explore a wide range of metallicities and alpha elements partitionsand show how these parameters affect the Asymptotic Giant Branch phase.The corresponding isochrones will be soon available on-line.


Observations of the Ultraviolet-Bright Star Barnard 29 in the Globular Cluster M13 (NGC 6205)

Dixon, William

We have analyzed FUSE, COS, GHRS, and Keck/HIRES spectra of the UV-bright star Barnard 29 in M13.  Fits to the star's optical spectrum yield Teff = 20,000 ± 100 K and log g = 3.00 ± 0.01. Using modern stellar-atmosphere models, we are able to reproduce the complex shape of the Balmer Ha feature.  We derive photospheric abundances of He, C, N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Fe, Ni, and Ge.  Barnard 29 exhibits an abundance pattern typical of the first-generation stars in M13.  We see no evidence of significant chemical evolution since the star left the RGB; in particular, it did not undergo third dredge-up.  Previous workers found that the star's FUV spectra yield an iron abundance about 0.5 dex lower than its optical spectrum, but the iron abundances derived from all of our spectra are generally consistent with one another and with the cluster value.  We attribute this discrepancy to their use of model atmospheres with a finite microturbulent velocity and fitting routines that allow the stellar continuum level to be a free parameter in the fit.  By comparing our best-fit model with the star's optical magnitudes, we derive a  mass M*/Msun = 0.41 – 0.50 and luminosity log L*/Lsun = 3.25 – 3.34, depending on the cluster distance.  Comparison with stellar-evolution models suggests that Barnard 29 evolved from a ZAHB star of mass M*/Msun ~ 0.50, a value close to the boundary between the extreme and blue horizontal branches.


RAMSES II - Raman Search for Extragalactic Symbiotic Stars

Angeloni, Rodolfo

Symbiotic stars (SySt) are long-period interacting binaries composed of a hot compact star - generally a white dwarf (WD) - an evolved giant star, and a tangled network of gas and dust nebulae. SySts represent unique laboratories for studying several astrophysical phenomena and their reciprocal influence. They are also among the most promising candidates as progenitors of SNIa.Presently, we know 252 SySt in the Milky Way and 62 in external galaxies. However, these numbers are still in striking contrast with the predicted population of SySt in our Galaxy that, according to different estimates, may oscillate between 10^3 and a few 10^5. The actual SySt population is thus a key issue that needs further investigating.One of the reasons for this embarrassing discrepancy between SySt population synthesis models and observations also stems from the fact that the SySt group has been historically defined on the basis of purely spectroscopic criteria. Because of many other stellar sources mimicking SySt colors, no photometric diagnostic diagram has so far demonstrated the ability to unambiguously identify a SySt, thus making the recourse to costly spectroscopic follow-up still inescapable.The two intense Raman OVI bands at 6830Å and 7088Å, however, are so unique to the symbiotic phenomenon that their presence is considered a sufficient criterion for classifying a star as symbiotic, even when the donor star is not directly visible through optical spectroscopy. In this poster we present the early results from RAMSES II (Raman Search for Extragalactic Symbiotic Stars), a Gemini/GMOS Upgrade Project funded by AURA, which makes use of the Raman OVI 6830Å band as a powerful photometric tool to identify new SySt within and beyond the Galaxy.


Raman-Scattered O VI 6825, 7082Å and C II 7023, 7053Å in the Symbiotic Nova RR Telescopii

Heo, Jeong-Eun

We present a high-resolution optical spectrum of the symbiotic nova RR Tel obtained with MIKE at Magellan-Claytelescope. RR Tel is a wide binary system of a hot white dwarf and a Mira with an orbital period of a few decades,where the white dwarf is accreting through gravitational capture of some fraction of material shed by the Mira.We find broad emission features at 6825, 7082, 7023, and 7053Å, which are formed through Raman scattering of far-UV O VI 1032, 1038, C II 1036 and 1037 with atomic hydrogen. Raman O VI 6825 and 7082 features exhibit clear double-peaked profiles indicative of an accretion flow with a characteristic speed ~40km/s. Raman C II features have a simple Gaussian profile of speed ~20km/s. We perform a profile analysis of the Raman O VI and C II features by assuming that O VI emission traces the accretion flow around the white dwarf with a representative scale of 1 AU whereas C II is formed in a much more extended region. A comparison of the restored fluxes of C~II 1036 and 1037 from Raman 7023 and 7053 and observed C~II 1335 an 1336 is consistent with the distance of RR~Tel $\sim 2 {\rm\ kpc}$ based on interstellar extinction of C~II.


Grid of realistic CSPNs atmosphere models

Fierro-Santillán, Celia

We present a grid of stellar atmosphere models, covering the region of the Central Stars of Planetary Nebule (CSPNs) in the H–R diagram. The models were calculated using the stellar atmosphere code CMFGEN ( Hillier and Miller 1998) in  ABACUS  supercomputer . The parameter space has 6 dimensions: surface temperature of the star, also called the effective temperature (Teff ), luminosity (L), metallicity (Z), and three stellar wind parameters, the exponent (ß) of the wind velocity law, the terminal velocity (Vinfinity), and the volume filling factor (Fcl ). The input parameters to the models were taken from the evolutionary traces of Miller Bertolami (2017), producing realistic models of atmospheres, which can be used as input to photoionization codes such as CLOUDY (Ferland et al. 1998)


TIGvival -- High-resolution spectroscopic monitoring of long-period variables using TIGRE

Wolter, Uwe

Since 2013, we have been carrying out a longterm spectroscopic monitoring campaign of long-period variables (LPV). We use the TIGRE 1.2 meter robotic telescope, located in La Luz, central Mexico. We obtain low-noise, high-resolution spectra covering the whole optical regime from 3800 Å to 8800 Å with a spectral resolution of 20000.Our target sample currently consists of 7 LPVs with periods between 90 and 400 days, Mira is one of them. The targets were chosen to represent different chemical and variability properties. To date, we have collected more than 300 spectra that evenly sample the cycles of our targets, as far as ground-based observations allow.Here, we present an overview of our dataset and preliminary results. We demonstrate the data quality and illustrate TIGvival's potential to help gauging future 3D simulations. The acronym TIGvival (TIGRE vigila variables de largo periodo a largo plazo = TIGRE long-term monitoring of LPVs) is meant to illustrate the long-term nature of our project: we plan to continue TIGvival for at least ten more years.


Studying the large-amplitude optical variabilty of V648 Car

Palma, Tali

Since 2012 we have been conducting, at the Swope telescope of the Las Campanas Observatory in Chile, an observing campaign aimed at searching for rapid photometric variability (i.e., flickering) in different types of accreting binary stars. V648 Car was recognize as a priority target among symbiotic stars - binaries composed of a hot compact star and a late-type giant - due to its supposedly high-mass WD and its hard X-ray emitting nature. After the first serendipitous discovery of V648 as a high-amplitude flickerer (Angeloni et al. 2012), a large amount of photometric data have been collected over the years with the aim of studying possible periodicities and thus further characterize the nature of the accreting component and of the binary system as a whole. In this work we present the preliminary results of this ongoing investigation.


A systematic survey of grain growth in discs around post-AGB binaries with PACS and SPIRE photometry

Dsilva, Karan

It is now well established that post-AGB binary systems are often surrounded by long-lived, stable circumbinary discs. The spectral energy distributions (SEDs) of these systems show a near-IR excess, making it a powerful tool to shortlist potential evolved binary systems. Interferometric studies have confirmed the very compact nature of the circumstellar material and infrared spectroscopy shows the dust grains in the disc are highly processed. Interaction between the circumbinary disc and the central system might impact the evolution of the system, and hence it is important to understand the evolution of the disc._x000D_ _x000D_ Previous studies of individual post-AGB binaries have shown that large grains are required to model the observed sub-millimetre flux. Here we present a systematic survey of 50 galactic post-AGB binary systems. Using data from Herschel (PACS/SPIRE), we extend the SEDs to sub-millimetre wavelengths and use the slope of the SED (called the spectral index) as a diagnostic tool to probe the presence of large grains. We observe a large diversity in spectral indices which means that we observe discs with varying degrees of dust processing. Using a Monte Carlo radiative transfer code (MCMax), we create a large grid of models to quantify the observed spectral indices. Our models include a self-consistent treatment of scattering as well as dust-settling._x000D_ _x000D_ The present study presents a uniform analysis of the diverse circumbinary discs around post-AGB binary systems and uses the presence of large grains in the disc as a proxy for evolution._x000D_ _x000D_ Keywords: stars: AGB and post-AGB – circumstellar matter – binaries: general – techniques: photometric – infrared: stars


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