Symposium 343 - Abstracts
Nucleosynthesis in AGB stars
Karakas, Amanda
The chemical evolution of the Universe is governed by the nucleosynthesis contribution from stars, which in turn is determined primarily by the initial stellar mass. I will review the status of theoretical stellar nucleosynthesis models and yields of single stars up to about 10 solar mass. Stars in this mass range evolve to become cool red giants after the main sequence, with the richest nucleosynthesis occurring during the AGB phase. It is during the giant branches that these stars experience mixing events that change the surface composition, with significant enrichments in carbon and heavy elements synthesized by the slow neutron capture process (the s-process). While the qualitative picture of the s-process is well known, there are major uncertainties that affect stellar yields and our understanding of the chemical evolution of galaxies. I will discuss some of these uncertainties and also highlight areas where progress has been made.
AGB Variables in Nearby Galaxies
Whitelock, Patricia
Certain types of large amplitude AGB variable are proving to be powerful distance indicators that will rival Cepheids in the JWST era of high precision infrared photometry. These are predominantly found in old populations and have low mass progenitors. At the other end of the AGB mass-scale, large amplitude variables, particularly those undergoing hot bottom burning, are the most luminous representatives of their population. These stars are losing mass copiously and are vital to our understanding of the integrated light of distant galaxies as well as to chemical enrichment. However, the evolution of such very luminous AGB variables is rapid and remains poorly understood. In this talk I will discuss recent infrared observations of both low- and intermediate-mass Mira variables in the Local Group and beyond.
Structure and Evolution of AGB stars: Remaining Challenges
Lattanzio, John
The basics of the structure and evolution of AGB stars have been known for some time. Yet a reliable quantitative understanding remains elusive. There are two main reasons for this. 1) the AGB phase is the last nuclear phase of low and intermediate mass stars, and hence uncertainties in the earlier evolution produce uncertainties in the structure of AGB models; 2) the complex physics occurring in AGB stars provides its own limitations on the models. In this presentation I will outline the main physics responsible for our lack of reliable models, and also discuss possible ways beyond the current impasse.
ON THE ONSET OF DUST FORMATION IN AGB STARS
Gobrecht, David
A promising candidate to initiate dust formation in oxygen-rich AGB stars is alumina (stoichiometric formula Al2O3) showing a spectral emission feature around ~13 µm attributed to Al-O stretching and bending modes (Posch et al. 1999, Sloan et al. 2003). The counterpart to alumina in carbon-rich AGB atmospheres is the highly refractory silicon carbide (SiC) showing a characteristic feature around 11.3 µm (Treffers & Cohen 1974). Both, alumina and SiC presolar grains are found in pristine meteorites with typical sizes of a few tens of nm to µms (Stroud et al. 2004). Owing to their refractory nature (thermal stability) and the large abundances of molecular gas-phase precursors (e.g. AlOH and SiC2), alumina and SiC grains are thought to represent the first condensates to emerge in the atmospheres of AGB stars. We follow a bottom-up approach, starting with the smallest stoichiometric clusters (i.e. Al4O6 Si2C2), successively building up larger-sized clusters. We present new results of the quantum-mechanical structure calculations of (Al2O3)n and (SiC)n clusters with n=1-12, including potential energies, rotational constants, charge distributions and structure-specific infrared spectra. The resulting vibrational spectra of the clusters is compared with observations and laboratory experiments. With the help of a vibration analysis we construct an appropriate partition function. With the latter, we evaluate thermodynamic potentials (enthalpy, entropy and Gibbs free energy) of the clusters under circumstellar conditions (p=10-5-10 Pa, T=500-6000 K) that differ significantly from standard conditions in the laboratory (p=105 Pa, T=298 K) and also from bulk potentials. Finally, we compute and present kinetic reaction rates describing growth and destruction of the clusters. The rates are implemented in an extensive chemical-kinetic network that is applied to trajectories describing the pulsation-driven mass loss in AGB circumstellar envelopes.
Planetary nebulae, morphology and binarity, and the relevance to AGB stars
Sahai, Raghvendra
The dramatic transformation of the spherical outflows of AGB stars into the extreme aspherical geometries seen during the planetary nebula (PN) phase is widely believed to be linked to binarity and is likely driven by the associated production of fast jets and central disks/torii. The key to understanding the engines that produce these jets and the jet-shaping mechanisms lies in the study of objects in transition between the AGB and PN phases. In this talk, I first briefly summarize results from the imaging surveys of large samples of planetary and pre-planetary nebulae with HST. Almost all objects have bipolar, multipolar and elliptical morphologies, and there is widespread presence of point-symmetric structure. I present results from studies of individual objects that highlight observational techniques being used to (a) determine the ages and momenta of the jet outflows which can be used to constrain the properties of the jet engines, and to (b) provide evidence for binarity during the AGB phase. Acknowledgements: The author's contribution to this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.
AGBs in binaries: Common envelope evolution
De Marco, Orsola
The common envelope binary interaction takes place when an expanding star transfers mass to a companion in an unstable manner leading to the companion entering the envelope of the giant. It leads to the ejection of the giant envelope and the emergence of a compact binary or to a merger. It also acts to interrupt the giant life, ejecting whatever its envelope contains into interstellar space, and producing an object (binary or single star) that would not exist in the absence of the interaction - like helium white dwarfs, CVs or type Ia supernovae, to mention a few. Both low mass companions and planets merging with AGB stars via a common envelope may alter profoundly the AGB star, including promoting nucleosynthesis and dredge up. Ultimately most of what we want to know about the common envelope interaction is in the hand of hydrodynamic models, which are themselves at the moment very limited in their ability to predict the various interesting effects we would like to know about. I will give an account of the latest advances in our knowledge of the common envelope interaction from theoretical and observational standpoints, as well as the impact of this phase in the life of galaxies.
Molecular dust precursors in envelopes of oxygen-rich AGB stars and red supergiants
Kaminski, Tomasz
Condensation of circumstellar dust begins with formation of molecular clusters close to the stellar photosphere. These clusters are predicted to act as condensation cores at lower temperatures, i.e. allow efficient dust formation farther away from the star. The condensation process is poorly understood, particularly in oxygen-rich stars. Recent observations of exotic metal oxides, such as AlO, AlOH, TiO, and TiO2, whose emission can be traced at high angular resolutions with ALMA, have allowed first observational studies of the condensation process in oxygen-rich stars. We are now in the era when depletion of gas-phase species into dust can be observed directly. I am going to present the most recent observations that allow us to identify the species that initiate the formation of inorganic dust close to photospheres of AGB stars and red supergiants. I am also going to discuss challenges one has to face in interpreting the observations, especially these related to non-equilibrium chemistry, non-LTE gas excitation, and high complexity of stellar atmospheres in the dust-formation zone. Finally, I am going to propose future observations and laboratory measurements that will help us overcome some of the difficulties.
LSST: making movies of AGB stars
Ivezic, Zeljko
LSST (www.lsst.org) will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachon in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 sq.deg. field of view, and a 3.2 Gigapixel camera. In a continuous observing campaign, LSST will cover the entire observable sky every three nights to a depth of V~25 per visit (using 30-second exposures and ugrizy filter set), with exquisitely accurate astrometry and photometry. Close to a half of the sky will be visited about 800 times during the nominal 10-year survey. The project is in the construction phase with first light expected in 2020 and the beginning of regular survey operations by 2022. I will describe how these data will impact AGB star research and discuss how the system could be further optimized by utilizing narrow-band TiO and CN filters.
The role of AGB stars in the evolution of Globular Clusters
Ventura, Paolo
It is now generally accepted that Globular Clusters harbour at least two groups of stars, differing in the chemical composition, particularly in the relative distribution of the species lighter than silicon. Because these chemical differences are not detected in field stars, these results indicate a self-enrichment mechanism, active in Globular Clusters. The search for possible polluters of the intracluster medium has stimulated a lively, still in progress debate. In this contribution I will discuss the various scenarios so far proposed and focus on the possibile role of massive AGB stars in the production of the nuclearly processed gas, from which the stars with the anomalous chemical composition might have formed.
3D-modelling of AGB stars
Freytag, Bernd
Local three-dimensional radiation-hydrodynamics simulations of patches of the surfaces of solar-type stars, that are governed by small-scale granular convection, have helped analyzing and interpreting observations for decades. These models contributed a lot to the understanding of the atmospheres and indirectly also of the interiors and the active layers above the surface of these stars. Of great help was of course the availability of a close-by prototype of these stars -- the sun. In the case of an AGB star, the convective cells have sizes comparable to the radius of the giant. Therefore, the extensions of the solar-type-star simulations to AGB stars have to be global and cover the entire object, including a large part of the convection zone, the molecule-formation layers in the inner atmosphere, and the dust-formation region in the outer atmosphere. I will present a new generation of CO5BOLD simulations and show how the interplay of large and small convection cells, waves, pulsations, and shocks, but also molecular and dust opacities of AGB stars create conditions very different from those in the solar atmosphere.
The evolution of DARWIN - the current status of wind models for AGB stars
Bladh, Sara
The slow, massive winds observed in evolved asymptotic giant branch stars are usually attributed to a combination of effective dust formation in the dynamic inner atmosphere and momentum transfer from stellar photons interacting with the newly formed dust particles. Dynamical models of the atmosphere and wind calculated with the DARWIN code (Höfner et al. 2016), using this mass-loss scenario, successfully produce outflows with dynamical and photometric properties compatible with observations, for both C-type and M-type AGB stars (e.g. Eriksson et al. 2014, Bladh et al. 2015). In this talk I will present the recent development in the DARWIN wind models for both M-type and C-type AGB star. I will show results from a new large grid of M-type AGB star models, spanning a wide range in effective temperature, mass and lumiosity, as well as results from models for C-type AGB stars at low metallicities, compatible to those in LMC and SMC.
The mass-loss characteristics of AGB stars, an observational view
Ramstedt, Sofia
The massive outflows of gas and dust which characterize giant stars on the AGB, builds a cool circumstellar envelope readily observed at sub-millimeter wavelengths. Observations will give the amount of matter lost by the star, the wind velocity (in the case of spectral line observations), and, when the spatial resolution is sufficient, the wind evolution over time.To gain detailed insight into the mass-loss process, we study the nearby (within 1 kpc) stars. Through these investigations we aim to determine the best constrained wind properties available. By combining this with theoretical results, mass-loss estimates for more distant sources can also be significantly improved. ALMA has opened up new opportunities to study the winds of AGB stars. The DEATHSTAR project (www.astro.uu.se/deathstar) has mapped the circumstellar CO emission from ~50 nearby M- and C-type AGB stars. The data will initially be used to give a definitive mass-loss prescription for the sample sources, but the large-bandwidth observations opens for many different legacy projects. The current status and results will be presented.
Magnetic fields of AGB and post-AGB stars
Vlemmings, Wouter
I will review the current status of magnetic field observations around AGB and post-AGB stars. I will shortly describe their possible role during these stages of evolution and will focus on our attempts to determine the origin of the magnetic field. Specifically recent efforts include ALMA observations of the near-stellar environment showing the signatures of hotspots and rotation.
Evolutionary timescales from AGB to PNe
Miller Bertolami, Marcelo
The transition from the asymptotic giant branch (AGB) to the formation of a planetary nebula (PN) is arguably one of the least understood phases of the evolution of low- and intermediate-mass stars. Recently we have computed a new set of post-AGB stellar evolution sequences based on a detailed callibration of previous evolutionary phases and an updated treatment of the microphysics. Interestingly, the new post-AGB models evolve three to ten times faster than predicted by the stellar evolution models that have been widely in use for more than two decades ---i.e. Vassiliadis & Wood (1994) and Bloecker (1995). This result holds for the whole mass and metallicity range. The new models are also ~0.1-0.3 dex brighter than the previous models with similar remnant masses. In this talk we will discuss the main differences with the older grids and try to shed some light on the reasons behind these differences. Finally, we will discuss some consequences of the new models for our current understanding of the formation of PNe, focusing on our current understanding of the planetary nebula luminosity function and the formation of planetary nebulae around low-mass central stars.
AGB stars and the cosmic dust cycle: a probe of galactic evolution
Zhukovska, Svitlana
Theoretical and observational studies of dust condensed in outflows from AGB stars have substantially advanced the understanding of the dust mixture from individual stars. This detailed information incorporated into models of the lifecycle of interstellar grains allows us to evaluate the contribution of AGB stellar populations to the galactic dust budget, their role in the dust production and how it changes with galactic evolution. I will discuss what we learn from applications of the models of the lifecycle of grains from AGB stars for galaxies near and far. I will highlight important questions of galactic evolution that can be answered by these models in combination with modern infrared observations.
AGB populations as probes of galaxy structure and evolution
Javadi, Atefeh
The evolution of galaxies is driven by the birth and death of stars. AGB stars are at the end points of their evolution and therefore their luminosities directly reflect their birth mass; this enables us to reconstruct the star formation history. These cool stars also produce dust grains that play an important role in the temperature regulation of the ISM, chemistry, and the formation of planets. These stars can be resolved in all of the nearby galaxies. Therefore, the Local Group of galaxies offers us a superb near-field cosmology site. Here we can reconstruct the formation histories, and probe the structure and dynamics, of spiral galaxies, of the many dwarf satellite galaxies surrounding the Milky Way and Andromeda, and of isolated dwarf galaxies. It also offers a variety of environments in which to study the detailed processes of galaxy evolution through studying the mass-loss mechanism and dust production by cool evolved stars. In this talk, I will first review our recent efforts to identify mass-losing AGB stars and RSGs in Local Group galaxies and to correlate spatial distributions of the AGB stars of different mass with galactic structures. Then, I will outline our methodology to reconstruct the star formation histories using variable pulsating AGB stars and RSGs and present the results for rates of mass-loss and dust production by pulsating AGB stars and their analysis in terms of stellar evolution and galaxy evolution.
Circumstellar dust, IR spectroscopy, mineralogy
Suh, Kyung-Won
In this work, we review IR spectroscopic studies of the circumstellar dust grains around AGB stars. The expanding envelopes of AGB stars are chemically fresh because of the strong binding force of CO molecules. O-rich dust grains (silicates, Al2O3, water ice, and other oxides) form in the O-rich envelopes and C-rich dust grains (amorphous carbon and SiC) form in the C-rich envelopes. Amorphous silicate grains could be efficiently crystallized by annealing processes in various environments of AGB stars. We also discuss dust mineralogy for objects that have undergone chemical transition processes.
Post-AGB and Post-RGB Stars as Tracers of Binary Evolution.
Kamath, Devika
It is well established that binary interactions can alter the intrinsic properties of stars (such as: pulsation, mass-loss, photospheric chemistry, dust-formation, circumstellar envelope morphology etc.) and play a dominant role in determining its ultimate fate. While past studies have shown that binarity can end the AGB life of a star, recent studies have revealed that, in specific cases, binarity can also pre-maturely terminate the RGB evolution. A characteristic feature of these evolved binaries is the presence of a Keplerian circumbinary disk of gas and dust which plays a lead role in the evolution of the systems. In this talk I will review our advances in the research landscape of post-AGB and post-RGB binary stars, focussing on their observational properties, spectral energy distribution, photospheric chemistry, the orbital parameters, the characterisation and evolution of their stable circumbinary discs, and the evolutionary connection between the enigmatic post-AGB and post-RGB binaries, and other systems whose primary component is a White Dwarf.
Circumstellar dust, interferometry and resolved circumstellar envelopes"
Lagadec, Eric
With adaptive optics and optical/IR interferometry, we can now map circumstellar envelopes down to the dust formation radius, and even map the surface of the closest AGB stars. This can give us clues on the mass-loss mechanism for these stars, by detecting convective cells, shocks and dust formation. The morphology of the resolved envelopes can also give us indirect evidence for the presence of a companion that will affect the shaping of the envelope.In this review I will present the latest results obtained with instruments such as SPHERE/VLT, Gravity and AMBER at the VLTI.I will also show how MATISSE, the latest addition to the VLTI, should revolutionize our view of dust formation around these stars. With a resolution down to 3 mas in the mid-infrared L band and observations in the N band, we will map MOLSPHERE and the dust formation zone with the best accuracy ever achieved.
The role of AGB stars in Galactic and cosmic chemical enrichment
Kobayashi, Chiaki
Elemental abundances and isotopic ratios of stars in the Galaxy have provided stringent constraints on the formation and evolutionary history of the Milky Way Galaxy. It is now possible to apply this Galactic archaeology approach for other galaxies where elemental abundances of stellar populations are measured within galaxies with IFU surveys. Using my galactic chemical evolution models and chemodynamical simulations, I will show AGB stars are important for the production of C, N, and s-process elements, while super-AGB stars are not because of the mass range. Electron capture supernovae can provide Sr, Zr, and Y, but are not the main source of r-process elements such as Eu. The production of C and N by AGB stars is important for constraining the star formation histories of disk galaxies and JWST will deliver such observations at high-redshift galaxies.
The population of planetary nebulae in the Galaxy and in nearby galaxies
Stanghellini, Letizia
Planetary nebulae (PNe) are the gaseous remnant of AGB stars, and they carry memory of their progenitor's history. In this review we focus on the population of Galactic and extragalactic PNe as probes of galactic evolution. We will discuss (i) PN populations across galaxy types; (ii) how to date PN progenitors; (iii) clues on the Galactic structure, based on PN probes; and (iv) radial metallicity gradients from Galactic and extragalactic PNe.
Resolved stellar populations: the outlook for JWST and ELT
Tolstoy, Eline
I will give an overview of the future of resolved stellar population studies of nearby galaxies as we move towards the era of JWST and ELT.I will discuss the science capabilities of two major future facilities, ELT and JWST, and how they can be utilised to make major break throughs in our understanding of important processes in galaxy evolution through the study of individual stars. They will both add extraordinary new detail to the resolved studies of nearby galaxies. As the telescopes get larger and more sensitive the local volume that can be resolved into individual stars also gets larger, and thus includes a wider range of different galaxy types. Deep resolved colour-magnitude diagrams will allow us to probe the star formation histories of systems that hither to have only been studied in integrated light, or only their bright young stellar populations have been resolved. We can hope to obtain deep and detailed observations of bulges and disks in a range of different type and mass of spiral galaxies. We can even hope to resolve individual luminous red and asymptotic giant branch stars in giant Elliptical galaxies at the distance of Virgo. We will also be able to complement the imaging of resolved stellar populations with spectroscopy of ever more distant individual stars.
The Impact of AGB Stars on Galaxies
Boyer, Martha
At the end of their evolution, asymptotic giant branch (AGB) stars undergo strong pulsation, mass loss, and dust production. Their mass loss results in substantial chemical and dust enrichment of the interstellar medium. Dust evolution models and isotope abundances in presolar grains suggest that AGB stars play a key role in both dust evolution and the star formation process. They are also the brightest stars in galaxies, potentially dominating in the near-infrared. As a result, AGB stars have a significant influence on the evolution and appearance of their host galaxies and thus must be accounted for when interpreting a galaxy’s integrated light. I will review the current understanding of the impact AGB stars have on galaxies, highlighting new observations of AGB stars in nearby galaxies and recent advances in modeling. Finally, I will comment on how future observations with upcoming telescopes could impact our understanding of this important phase of stellar evolution.
Lumpy stars and bumpy winds
Liljegren, Sofie
The wind-driving process of AGB stars are thought to be a two-step process: first matter is levitated by shock waves, and then accelerated outwards by radiation pressure on newly condensed dust grains. When modelling such a wind, spherical symmetry is usually assumed. This is in stark contrast with recent observations, which shows significant non-spherical structures. Giant convection cells cover the surface of the star, and matter is being ejected into the atmosphere where it condenses into lumpy dust clouds. We try to quantify the differences between what is simulated in the 3D star-in-a-box models (CO5BOLD code) and the 1D dynamical atmosphere and wind models (DARWIN code). The impact of having a non-spherical star on the wind properties is also investigated. We find that the inherent non-spherical behaviour of the AGB stars might induce a dust-driven weak wind already early on the AGB, and including that the star is anisotropic when simulating the wind leads to large time variation of the density of the outflow. Such variations could possibly be observable as small-scale structures in the circumstellar envelope.
Chemistry and binarity in the early Universe: what is the role of metal-poor AGB stars?
Arentsen, Anke
To research the earliest times in the Universe we do not have to go to high redshift. Our Milky Way still hosts remnants from these times in the form of metal-deficient stars, which probably carry the imprint from First Star supernovae. However, if they were born in binary systems, their chemical patterns today can also be influenced by a companion going through an AGB phase. Disentangling these two enrichment scenarios will progress our understanding of both First Stars and AGB star chemistry at extremely low metallicities. At the lowest metallicities, the fraction of stars that are strongly enhanced in carbon dramatically increases. These carbon-enhanced metal-poor (CEMP) stars come in two main types: CEMP-s stars show additional enhancement in s-process elements, CEMP-no stars do not and occur at even lower metallicities. CEMP-s stars are thought to have received their carbon and s-process elements in mass-transfer from an AGB companion that has evolved earlier. This has been supported by radial velocity monitoring (e.g. Hansen et al. 2016a) which found many (but not all) of these stars to be in binaries. The CEMP-no stars - of which the majority show no velocity variations - would instead reflect the chemistry of their birth gas cloud. Although CEMP-no stars are not typically in binary systems, some of them do show radial velocity variations consistent with an unseen companion. This provokes the question if some extremely low-metallicity AGB companions (currently white dwarfs) could have polluted their atmospheres. In this presentation I will focus on the results of a large extension of our radial velocity monitoring program for CEMP-no stars. Compared to earlier work (Starkenburg et al. 2014; Hansen et al. 2016b), the number of known CEMP-no binaries has now more than doubled. An interesting pattern starts to emerge, raising questions about the origins of CEMP-no stars and their possible relation to (extremely metal-poor) AGB stars.
On the circumstellar envelopes of semi-regular long-period variables
DIAZ-LUIS, JOSE JAIRO
The mass loss process along the AGB phase is crucial for the formation of circumstellar envelopes (CSEs), which in the post-AGB phase will evolve into planetary nebulae (PNe). This transformation is relatively well known since many years, however, there are still important issues that would need to be further explored; in particular, the formation of axially symmetric PNe from spherical CSEs. Recent studies show that semi-regular variables (SRs) appear to have axi-symmetric envelopes, what implies that they lose mass in a different way than other evolved stars (e.g., Mira variables). In order to address the problem, we have conducted high S/N IRAM 30m observations of 12CO (J=2-1, 1-0) and 13CO (J=1-0), as well as Yebes 40m SiO (J=1-0) observations, in a volume limited sample of SRs. The CO molecule is the best tracer for total gas and provides a good description of the envelope structure, while SiO is expected to arise from the innermost regions of the envelopes. The results suggest a large prevalence of non-standard envelopes in these sources. We have characterized the main properties of the CSEs, such as expansion velocity, mass loss rate, wind density, etc. We have also explored possible relationships between these properties and the main stellar properties (e.g., effective temperature, pulsation, etc.). Interestingly, we report a very strong correlation between mass loss rates and the 12CO(1-0)-to-12CO(2-1) intensity ratio of the sample, introducing a new method for determining mass loss rates of SRs with short analyses time. We also find that for several stars the SiO profiles are very similar to those of the CO profiles, a totally unexpected result. This is not viable unless these are non-standard envelopes.
What Young Massive Clusters in the Magellanic Clouds teach us about Old Galactic Globular Clusters?
D'Antona, Francesca
Recent work on the Young Massive Clusters (YMC) of the Magellanic Clouds (MCs) has shown that rapid rotation, together with its slow down due to the evolution, plays an important role in shaping the color magnitude diagram features, and in particular the presence of an extended turnoff region and of a "split" main sequence, whose bluer side is populated by slowly rotating stars, and the redder side is populated by initially rapidly rotating stars. Thus the phenomenon of multiple populations in these young (age < 400Myr) clusters seems to be unrelated to the features of multiple populations in old Galactic Globular Clusters. In this talk, we remind that the Asymptotic Giant Branch (AGB) scenario explains these latter multiple populations by different episodes of star formation occurring from gas contaminated by the ejecta of massive AGBs and super-AGBs of a first stellar population. These AGBs are then stars as massive as 4-8Msun, those which today populate the MCs YMC. Consequently, we must pose the question whether the evolution of such massive AGBs has been influenced by rotation, how much this affects the yields, and whether the resulting abundances are compatible with the chemical patterns observed and with color magnitude properties such as the morphology of the horizontal branch. We show the first promising results of this analysis.
The End: Watching the Death of Extreme Carbon Stars in the Large Magellanic Cloud
Sloan, G.C.
Extremely Red Objects (EROs) are highly evolved carbon stars embedded deeply in their own dust. Several were discovered in the Large Magellanic Cloud by the Spitzer Space Telescope. We appear to have caught them in the moment of transition off of the asymptotic giant branch. Multi-epoch observations from Spitzer and the Wide-field Infrared Survey Explorer (WISE) reveal weak variability, in contrast to the trend of increasing pulsation amplitudes with more dust as seen in most carbon stars. The photometric data also reveal excess blue emission at shorter wavelengths, which would be expected as the stellar envelope detaches from the remnant core and the dust shell moves outward and begins to deviate from spherical symmetry. In this scenario, the blue excess could arise from light scattered from regions in the extended dust shell with more direct lines of sight to the central star. These objects give us the chance to study, in detail, how a star begins evolving into a planetary nebula. We have begun a campaign to look more closely. Initial results with adaptive-optics imaging of the dustiest ERO confirm the expected asymmetry at sub-arcsecond scales.
Extended dust emission from nearby Asymptotic Giant Branch stars
Dharmawardena, Thavisha
Asymptotic Giant Branch (AGB) stars in the Milky Way are often overlooked due to difficulties in obtaining distances. Understanding their dust emission properties and mass-loss histories leads to better comprehension of the ISM evolution in the Milky Way, in particular its dust reservoir. Most studies have determined the stellar dust mass using mid-IR emission, but a historic cold dust reservoir from a previous mass-loss phase may remain undetected in the mid-IR, requiring observations at longer wavelengths.We present results of a search for extended dust emission from a diverse sample of AGB stars within 1 kpc. This project is part of the Nearby Evolved Star Survey (NESS), which aims to determine gas and dust return from a volume limited sample of galactic evolved stars.Observations at 450µm and 850µm were carried out using the SCUBA-2 instrument on the JCMT and combined with Herschel PACS observations at 70µm and 160µm from the MESS survey (Groenewegen et al., 2011; Cox et al., 2012). Using azimuthally-averaged surface brightness profiles we determined extension and percentage flux of the extended component for each source at the four wavelengths. We derived extensions at the 3s levels greater than ~40'' for majority of the sources at SCUBA-2 850µm. The fraction of the flux emitted from the extended region is ~40% at 850µm and ~50% at PACS wavelengths.By fitting a modified black body to four-point SEDs at each radial point we derive the dust temperature, spectral index of emissivity, and column density to probe dust mass-loss history and detect changes in physical properties of dust as a function radius (hence, time). The resulting dust-to-gas ratios are consistent with canonical values. The surface density profiles suggest deviation from constant mass loss in a majority of the sources.Here, we report a clear detection of the detached shell of U Ant in both 450µm and 850µm. This is the first time this detached shell has been detected in the sub-mm continuum.
Newly discovered Planetary Nebulae population in Andromeda (M31): PN Luminosity function and implications for the late stages of stellar evolution
Bhattacharya, Souradeep
Stars with masses between ~0.7 and 8 M? end their lives as Planetary Nebulae (PNe) following the AGB phase. Because of their relatively strong [OIII] 5007Å emission, PNe can be readily identified in continuum-subtracted narrowband images, and their spectroscopic follow up provides information on the kinematics and oxygen abundance in regions where the surface brightness of the parent stellar population is too faint for absorption line spectroscopy. With the MegaCam@CFHT, we have carried out a survey of the inner central 24 sq. degrees of Andromeda (M31) reaching the outer disk and halo, using a narrow-band filter centered on the [OIII] 5007Å emission line and a broad-band g filter. This survey extends the previous PN sample by Merrett et al. 2006 (MNRAS, 369, 120) both in uniform area coverage and depth. It promises to answer the open questions on the kinematics of the stellar halo in Andromeda galaxy and the shape of the PN luminosity function (PNLF) at the faint end. From the continuum-subtracted narrowband images, we identify ~4000 PNe in M31, of which ~3000 are newly discovered. We obtain PNe down to ~6 mag below the bright cut-off of the PNLF, ~2 mag deeper than in previous works. We find that the bright cut-off of the PNLF is consistent with previous determinations, however, we are now able to detect a steep rise in the number of PNe ~5 mag fainter than the bright cut-off. This steep increase persists in the PNLFs obtained at different projected elliptical radii in M31, and is reminiscent of the morphology of the PNLF measured for the Large Magellanic Cloud (Reid & Parker 2010, MNRAS, 405, 1349). The next steps include the association of the PN population with the RGB stars from the Pan-Andromeda Archeological Survey (PAndAS) and discussion of possible causes of the steep rise at the faint end of the PNLF in the framework of the post-AGB stellar evolution models.
The impact of UV radiation on the circumstellar chemistry
Saberi, Maryam
Penetration of ultraviolet (UV) radiation from the interstellar medium controls the chemistry in the outer part of the circumstellar envelope (CSE) around evolved stars. However, recent GALEX observations revealed 180 asymptotic giant branch (AGB) stars (57 % of the observed sample) have detectable Far- and/or Near-UV emissions, indicating the presence of extra UV radiation inside the CSE. The internal UV radiation can originate from stellar chromospheric activity, a hot binary companion, and/or the accretion of material between two stars in the binary systems.To constrain the effects of both internal and external UV radiation in the chemical modeling of CSEs, observations of main photodissociation/photoionization products are required. Moreover, comparison of isotopologue ratios of molecular species such as 12CO/13CO and H12CN/H13CN which dissociate through different mechanism can be used to trace the UV-chemistry. I will present our approach to studying the effect of UV-chemistry in the CSEs based on the ALMA and APEX observations and chemical modeling. I will also show the first CI detection towards the Mira AB system. Finally, I will discuss how this method can be used to indirectly detect unidentified hot binary companions of AGB stars.
Common envelope evolution and the role of accretion
Chamandy, Luke
Binarity is likely needed to explain the ubiquity of bipolarity observed in planetary nebulae (PNe) and pre-planetary nebulae (pPNe). The central stars of several bipolar PNe have been observed to be close binaries, with the symmetry axis of the PN perpendicular to the orbital plane. Such systems likely experienced a common envelope interaction, wherein the core of the original AGB star and its companion rapidly spiralled in toward one another, and ejected the envelope. This process remains poorly understood, largely because it is inherently 3D, involves a vast range of scales, and is computationally demanding. Simulations have yet to produce a tight binary and ejected envelope when only the binary potential energy is used to propel the envelope, suggesting additional power sources might be necessary. One such source is accretion onto the companion (e.g. a white dwarf or main sequence star). We have performed fully 3D global hydrodynamical simulations of common envelope evolution in high resolution using the AMR code AstroBEAR, and bracket the plausible range of accretion rates from the results of two such simulations---one with a subgrid accretion model based on the Bondi-Hoyle-Lyttleton formalism, and the other without any subgrid accretion. From this exploration we argue that super-Eddington accretion onto the companion may be common. This could power jets which help to unbind and shape the envelope during inspiral.
The Composition of Barium stars and the s-Process in AGB Stars
Cseh, Borbála
Barium stars belong to a binary system where the companion star has evolved through the AGB phase and transferred elements heavier than Fe produced by the slow neutron-capture process onto the secondary star, which is now observed. A new large set of homogeneous high resolution spectra of Barium stars makes it now possible to meaningfully compare the observational data with different AGB models. We performed a new detailed error analysis of these observations and demonstrate for the first time that the data shows a clear increase of the hs-type/ls-type element ratio (for example, Ce/Y) with decreasing the metallicity, as predicted by non-rotating AGB models where 13C is the main neutron source. We conclude that rotational mixing is a minor effect. Rotating cores on the AGB slower than expected if the angular momentum is conserved may derive from coupling between the core and the envelope. The existence of such coupling has also been invoked to explain the low rotational velocities of the cores of red giant stars and of white dwarfs (the ancestors and the progeny of AGB stars, respectively) inferred via asteroseismology from Kepler observations.
Determining the Luminosity of the Third Dredge-up via S-type stars: the promise of Gaia
Shetye, Shreeya
S stars are late-type giants whose spectra show distinctive molecular bands with as most noticeable characteristic the appearance of ZrO bands. Their abundance patterns show signature of s-process nucleosynthesis, while the C/O ratio is intermediate between that of M-type giants and carbon stars. They can be segregated in two classes by probing their Tc content. The Tc-rich S stars are intrinsic S stars on the Asymptotic Giant Branch (AGB), producing the s-process elements internally which are then transported to the stellar surface via third dredge-up. The intrinsic S stars are the first ones on the AGB to have undergone a third dredge-up. Tc-poor S stars, in turn, are all members of binary systems which were enriched in carbon and s-process elements by the former AGB companion which is now a cold dim white dwarf.Getting a reliable Hertzsprung-Russell (HR) diagram for S-type stars is difficult not only because of their unconstrained distances but also because their complex atmosphere makes the determination of their stellar parameters challenging. We devised a spectral fitting method to disentangle the stellar parameters of S stars. The method makes use of MARCS model atmospheres for S-type stars covering a vast range in effective temperature(Teff), surface gravity, metallicity, [s/Fe], and C/O and comparing them with high-resolution, high S/N HERMES spectra. The accurate parallaxes from Gaia DR2 combined with the well-constrained Teff will provide the positions of the S stars on the evolutionary tracks in the HR diagram hence further constraining their masses and surface gravities.The parameter study along with the luminosity of intrinsic S stars will help us marking the onset of the third dredge-up in the HR diagram. The ultimate goal is to gain insight in the AGB nucleosynthesis and third dredge-up episodes in function of evolutionary mass and metallicity.
On the nature and mass loss of Bulge OH/IR stars
Blommaert, Joris
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.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.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.
The missing mass conundrum of post-common-envelope planetary nebulae
Santander-García, Miguel
Stars of 0.7 to 8 M$_\odot$ end their lives as planetary nebulae (PNe), by exposing their inert cores and ejecting their gaseous envelopes into the interstellar medium (ISM). Most PNe show beautiful, aspherical morphologies with high degrees of symmetry, despite their progenitor stars being essentially spherical. Angular momentum provided by a close binary companion has been widely invoked as the main shaping agent that would eject the nebula along preferred axes and planes: the engulfment of the companion by the Asymptotic Giant Branch (AGB) star into a common envelope (CE) would trigger a very brief phase (1-10 years) in which the orbit would shrink substantially due to drag forces, thus providing gravitational energy and angular momentum to eject this CE into a bipolar PN. The evolution of the AGB would be thus interrupted abruptly, its (still quite) massive envelope fully ejected to form the PN. In other words, PNe ejected through this process should be substantially more massive than those coming from single AGB stars, where most of that material was deployed into the ISM long ago, and is now too diluted to be detectable.Even though models succesfully predict the equator of these PNe to coincide with the orbital plane of the binary star —as occurs in every single case analysed so far—, they mostly fail on achieving escape velocities for the ejected material with the assumed angular momentum and gravitational energy reservoirs.We present observations of a pilot sample of confirmed post-CE PNe in the molecular regime, together with estimates of their total (atomic+molecular) mass content and linear momenta, in order to better understand their nature, and constrain models of formation. We find the nebular mass of our sample to be significantly lower than expected for an AGB envelope, leading us to wonder if models actually work for the actual masses involved, and also to raise some doubts on our understanding of the formation of these intriguing beasts.
Dynamics, temperature, chemistry, and dust: ingredients for a self-consistent AGB wind
Boulangier, Jels
For decades it has been known that the mass loss of evolved, low-mass giant stars is one of the major contributors to the chemical enrichment of galaxies. Yet, the actual mechanisms governing this mass loss still elude us. The general hypothesis is that the mass loss of these Asymptotic Giant Branch (AGB) stars is generated by a combination of pulsations and radiative pressure on dust grains. However, the physical and chemical conditions of AGB winds are poorly understood. Considerable effort has been made in modelling these winds dynamically and radiatively. Yet, all studies assume gas-phase chemical equilibrium.We have developed a self-consistent hydrochemical model, improving on the current wind models. This approach is needed because of the intricate interplay between dynamics and chemistry that is caused by shocks and cooling/heating processes. The chemical composition determines which cooling/heating mechanisms influence the thermal evolution of the gas. In turn, the temperature acts on the dynamical behaviour of the wind which again will impact the rate of chemical reactions. Therefore, we take into account most important chemical reactions and thermal mechanisms.Our initial results strengthen the hypothesis that dust is necessary to drive these stellar winds. They also predict that ideal dust formation conditions (i.e. cold and dense) could arise close to the star. Because of this, we've included kinetic nucleation theory for TiO2 and Al2O3, whose large clusters can act as seeds for dust formation. Hereby, we eliminate the need for assumptions on seed particle abundances and their position in the wind. The seeds can then grow to become larger dust grains via dust formation/destruction mechanisms. This seed-to-dust approach allows to more self-consistently predict abundances and grain size distribution across the stellar winds.This work lays the basis for dust-gas chemistry complemented with dust acceleration, striving for a more accurate AGB wind model.
AGB stars in Gaia Data Release 2
Lebzelter, Thomas
Gaia Data Release 2 (DR2; April 25, 2018) provides astrometric, photometric, and spectroscopic data for more than a billion stars - among them many AGB stars. As part of DR2 the light curves of several hundreds of thousand variable stars, including many Long Period Variable (LPV) candidates, are made available. The publication of the light curves and LPV-specific attributes in addition to the standard DR2 products offers a unique opportunity to study AGB stars.In this contribution, we present the first results for AGB stars based on the analysis of the Gaia data performed after their release. The analysis focuses on the AGB properties in connection to stellar distances, sky distribution and variability behaviour.
High-resolution observations of AGB stars with ALMA and SPHERE: imaging the stars, the gas, and the dust
Khouri, Theo
The mass-loss mechanism of asymptotic giant branch stars has long been thought to rely on two processes: stellar pulsations and dust formation. The details of the mass-loss mechanism have remained elusive, however, because of the overall complexity of the dust formation process in the very dynamical pulsation-enhanced atmosphere. Recently, our understanding of AGB stars and the associated mass loss has evolved significantly, thanks both to new instruments which allow sensitive and high-angular-resolution observations and the development of models for the convective AGB envelopes and the dust formation process. ALMA and SPHERE/ZIMPOL on the VLT have been very important instruments in driving this advance in the last few years by providing high-angular resolution images in the sub-mm and visible wavelengths, respectively. I will present observations obtained using these instruments in the same epoch (2.5 weeks apart) of two AGB stars, Mira and R Doradus, that resolve even the stellar disks. Moreover, ALMA reveals the distribution and dynamics of the gas around the star, while the polarised light imaged using SPHERE shows the distribution of the dust grains expected to drive the outflows. The inner circumstellar environments of the two stars differ significantly. On the one hand, the observations of Mira show a central source surrounded by asymmetric distributions of gas and dust, with complementary structures seen in the two components. The observations of R Doradus, on the other hand, reveal a much more symmetric inner envelope in which dust condenses efficiently very close to the star. The well-resolved stellar disk of R Dor is asymmetric, however, and appears different in the visible and the sub-mm. By fitting radiative transfer models to high-excitation molecular lines and light scattered by dust, the gas and dust density distributions are constrained to provide important constraints to wind-driving models.
Abundances of C, N and O in AGB Giants and Model Atmospheres
Aringer, Bernhard
Atmospheric models of AGB giants are usually calculated with a scaled solar chemical mixture, except for carbon and maybe some s-process elements. However, due to the combination of nuclear processes in their interior and dredge-up events, also the surface abundances of nitrogen and oxygen might be significantly altered in those objects. We have produced a large number of C and M-type COMARCS models, where different C, N and O abundances are taken into account. Based on these computations we discuss the corresponding effects on synthetic spectra and photometry of individual stars, as well as possibilities to include them in population synthesis models for galaxies. We will for example show that for photometric studies it should be enough to consider the metallicity and [C-O] values of single objects.In addition, we discuss the possibilities of measuring an enhancement of oxygen or nitrogen, as well as the [C-O] or C/O values from a theoretical point of view, demonstrating the importance of a consistent treatment of opacities in the models and spectral synthesis and investigating the uncertainties connected to stellar parameters, dust obscuration, pulsation and massloss. It will be shown that with the models available today accurate abundance determinations are only possible for warmer and less pulsating AGB giants.
SiO masers in thousands of AGB stars in the Galaxy
Sjouwerman, Lorant
We report on the Bulge asymmetries and Dynamic Evolution (BAaDE) survey which has observed 19,000 MSX color selected red giant stars for SiO maser emission at 43 GHz with the VLA and is in the process of observing 9,000 of these stars with ALMA at 86 GHz in the southern sky. Our setup covers the main maser transitions as well as those of isotopologues and selected carbon lines. Observations of this set of lines allows a far-reaching catalog of line-of-sight velocities in the dust-obscured regions where optical surveys cannot reach. Our preliminary detection rate is close to 70%, predicting a wealth of new information on the distribution of metal rich and carbon rich stars, their kinematics as function of location in the Galaxy, as well as the occurrence of lines and line ratios between the different transitions in combination with the spectral energy distribution from about 1 to 100 micron. Similar to the OH/IR stars, a clear kinematic signature between disk and bulge stars can be seen. Furthermore, the J=1-0 (v=3) line plays a prominent role in the derived maser properties, indicating diverse and/or variable pumping conditions. We further present a possibility to separate carbon-rich from oxygen-rich AGB stars using MSX colors.
Dust formation around aluminum oxide-rich AGB stars: condensation experiments in the Al-Si-O system
Takigawa, Aki
Mid-infrared spectroscopic observations of dust shells around oxygen-rich asymptotic giant branch (AGB) stars indicated that materials emitting a broad peak at 11-12 µm abundantly coexist with amorphous silicate dust around many of these stars (Sloan et al. 2003). Amorphous aluminum oxide synthesized by a sol-gel method well reproduces that emission, but it is not obvious that amorphous aluminum oxide can condense around these stars because of its very high glass transition temperature. Possible compositions of amorphous materials reproducing a broad 11-12 µm peak other than pure aluminum oxide have not been examined. We carried out condensation experiments of Al-Si-O gases using the induction thermal plasma system (Kim et al 2017). Aluminum and silicon powders with various mixing ratios (only Al, Al/Si = 9, 3, 1, 0.08, and only Si) were used as starting materials. They were evaporated at high temperature in Ar-O2 plasma flames (~10000 K) and rapidly cooled (104~105 K/s). The condensed particles were analyzed with X-ray diffraction and Fourier transform infrared (FT-IR) spectroscopy, and observed with transmission electron microscope. The condensed nanoparticles from evaporated aluminum was d-alumina. Particles with structures similar to ?-alumina were observed in condensates from starting materials of Al/Si=9 and 3. Amorphous particles were also observed in samples with Al/Si<3 and no crystals was observed in condensates from samples with Al/Si<1. The FT-IR spectrum d-alumina shows multiple peaks on a broad feature at 11-14 µm, which is not observed in circumstellar dust emissions. Condensates from starting materials of Al/Si = 9 and 3 shows peaks at slightly longer than observed dust emission but well reproduces its spectral shape.These results indicate that the carrier of 11-12 µm broad emission is not pure aluminum oxide but amorphous materials containing 10-30% of other elements including Si.
A Tough Egg to Crack
lim, jeremy
A Tough Egg to CrackThe sculpting of the Egg proto-planetary nebula has so far defied a coherent explanation. This nebula exhibits prominent bipolar lobes, multiple equatorial outflows, and quasi-circular arcs imprinted on an approximately spherically-symmetric wind from the progenitor AGB-star. Here, we use observations at high angular resolutions to study gas and dust at wavelengths ranging from centimeter (VLA), millimetre (ALMA), to near-IR and optical (HST) wavelengths so as to determine the position of the previously mass-losing AGB star and present post-AGB star that illuminates the nebula. We find that: (i) dust close to the centre comprises elongated blobs aligned with the wall of the northern lobe, none of which coincide with the central illuminating star; (ii) gas near the systemic velocity traces an expanding, hollow, spherical shell - a last burp from the AGB star - disrupted by the bipolar outflows; (iii) the position of the post-AGB star as previously determined from polarized scattered light coincides closely with the centre of the hollow shell, the position of the previously mass-losing AGB star. We argue that the sculpting of the Egg proto-planetary nebula requires two stellar companions to the post-AGB star, and present a working model for how all the intricate structure of this nebula may be produced.
On the Peculiar Lives of Primordial AGB Stars
Campbell, Simon
Models of zero metallicity and extremely metal-poor stars show that they evolve differently to their more metal-rich counterparts. In particular they suffer violent episodes that very often lead to extreme carbon enrichment at the surface before or during the AGB phase. The large amounts of carbon in their atmospheres has a fundamental effect on their AGB evolution. The carbon enrichment may also be accompanied by s- or i-process products, thus making the stars intrinsic CEMP or CEMP-s/i stars. The AGB stars could also serve as sources for binary pollution. In this talk I will describe the evolution of a primordial star from the main sequence through to the TP-AGB and present an overview of the evolutionary outcomes we have found in our grid of Z=0 and EMP models. We have also calculated the AGB nucleosynthetic yields for these stars, with the caveat that they suffer from many uncertainties.
Investigating orbital and chemical properties of galactic post-AGB binaries
Oomen, Glenn-Michael
Post-AGB stars are transition objects between the AGB and PN evolutionary phases. One of the most important research questions regarding the final evolution of low- and intermediate-mass stars, is the impact of binarity. Binary interaction alters the intrinsic properties of the star (such as: pulsation, mass-loss, dust-formation, circumstellar envelope morphology, etc.) and plays a dominant role in determining its ultimate fate.In this contribution we focus on post-AGB binaries that are the direct result of an interaction on the AGB. We present our results of a homogeneous analysis of decade-long long radial velocity monitoring campaign of post-AGB binaries. These were specifically selected on the basis of their infrared dust excess, which shows the presence of a circumbinary disc. We derived orbital elements for no less than 33 galactic post-AGB binaries. All orbits have periods in the range 100 – 3000 days, often with non-zero eccentricity. These orbital properties cannot be predicted by population synthesis models, since the progenitors of the post-AGB stars must have filled their Roche lobes with the current observed orbital periods. This is not only thought to have circularised the systems, but is also expected to lead to spiral-in resulting in much shorter orbits. Neither circularisation, nor severe orbital shrinkage is detected.Furthermore, we investigate the phenomenon of depletion of refractory elements in the post-AGB photosphere, which is thought to be the result of accretion of metal-poor gas from the circumbinary disc. The ultimate aim of our research is to come to a better understanding of how these binaries evolve. Given the very strong discrepancy between the observed orbital characteristics and the predictions of detailed binary evolution models, this involves a systematic study and evaluation of new binary interaction processes, which are constrained and inspired by the results of a systematic observational programme.
Constraining convection across the AGB with high angular resolution observations
Paladini, Claudia
Convection plays a major prole in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars. Most of our knowledge about stellar convection comes from studying the Sun. On the surface of our star a couple of millions of convective cells are observed, each one with a size of about 2000 km. Following predictions dating back to the ’70, the surface of evolved stars (i.e. the AGBs) is expected to be populated by a few large convective cells several tens of thousand times the size of the solar ones. In this contribution I will present very detailed images of the photosphere of an AGB star obtained with the PIONIER instrument, mounted on the Very Large Telescope Interferometer. The images show a well defined stellar disc populated by a few convective patterns. Thanks to the high quality of our observations we are able to derive characteristic quantities such as contrast and granulation horizontal scale for the first time in a direct way. Such quantities are then compared with scaling relations between granule size, effective temperature and surface gravity that are predicted by simulations of stellar surface convection. I will conclude describing shortly what the next generation of interferometric instruments will bring to our study.
Hiding in plain sight? The search for the elusive super-AGB stars
Doherty, Carolyn
Super-AGB stars reside in the mass range ~ 6-12 Msun and bridge the divide between low/intermediate-mass and massive stars. They are characterised by off-centre carbon ignition prior to a thermally pulsing phase which can consist of many 10-1000s of thermal pulses. With their high luminosities and very large, cool, red stellar envelopes, these stars may appear seemingly identical to their slightly more massive red supergiant (RSG) counterparts and may act as massive star imposters in RSG surveys. However, as yet there has been no observationally confirmed super-AGB stars.Super-AGB stars undergo a variety of nucleosynthetic processes including proton-capture reactions at the base of the convective envelope and heavy element (s-process) production during the thermal pulses. The most massive super-AGB stars can also undergo a dredge-out event, where, a convective helium burning region merges with an inward moving convective envelope. When these zones meet, hydrogen is mixed down to very high temperature regions where a 13C rich region forms, leading to subsequent neutron release and heavy element (i-process) production. Also important for super-AGB stars is rotation, and in particular its impact to the surface composition relative to the process of second dredge up. The chemical surface enrichment from all of these processes has not yet been explored and may result in a clear nucleosynthetic signature to differentiate between super-AGB stars and (massive star) RSGs. The refining of this mass boundary has important implications for the energetics and chemical enrichment of galaxies.Here we present grids of rotating and non-rotating super-AGB star models along the entire TP phase and for (low mass) massive stars until the point of core collapse. These calculations span a broad range of metallicities from Z=0 to 0.04 an include both the first detailed heavy element nucleosynthesis for dredge-out events and super-AGB star nucleosynthesis including the impact of rotation.
The Role of TP-AGB and P-AGB Stars in Stellar Populations
Bruzual, Gustavo
Thermally pulsing AGB (TP-AGB) stars contribute a large fraction of the NIR emission in intermediate age stellar populations. Post AGB (P-AGB) stars have proven to be an important source of ionizing radiation in early type galaxies (e.g., Binette et al. 1994). These phases of stellar evolution thus must be treated carefully in population synthesis models aimed to explore the spectral properties of galaxies at different cosmic times. We present state of the art population synthesis models that improve on the following aspects with respect to most available models. (a) Treatment of the UV spectral range, including new evolutionary tracks and updated treatment of the ionizing radiation emitted by the stellar population, including a new treatment of P-AGB evolution and emission spectra. (b) Improved treatment of TP-AGB stars that dominate the NIR spectral range. The number of these stars present in a stellar population has been calibrated carefully by comparison with observations of the LMC/SMC, and the spectra assigned to these stars include both the stellar emission and the emission from circumstellar dust. (c) Definition of new diagnostics to characterize the combined stellar and nebular emission and the cold ISM in galaxies. Models are available in a wide range of metallicity, from Z = 0 to Z = 0.06, and are ready to use to interpret the spectra of galaxies of any age at low and high redshift.
Calibrating TP-AGB stellar models and chemical yields through resolved stellar populations in the SMC
Pastorelli, Giada
Most of the physical processes driving the TP-AGB evolution – mass-loss, occurrence and efficiency of third dredge-up – are not yet fully understood and they need to be modelled with parametrized descriptions. Recent photometric and spectroscopic surveys of resolved stellar populations in the Local Group allow us to perform a reliable calibration of the uncertain parameters, thanks to the detailed identification and characterization of TP-AGB stars and to the accurate measurement of the star formation history of the host systems. We present the results of the on-going calibration of the TP-AGB phase based on a complete sample of AGB stars in the Small Magellanic Cloud observed in the near- and mid-infrared wavelengths (SAGE-SMC survey). We computed large grids of TP-AGB evolutionary tracks with several combinations of third dredge-up and mass-loss prescriptions and we included them in the population synthesis code TRILEGAL, which simulates the photometry and the stellar parameters of a stellar system given its star formation history, distance and reddening. The synthetic TP-AGB population of the SMC is modelled according to the detailed space-resolved star formation history derived with deep near-infrared photometry from the VISTA survey of the Magellanic Clouds (VMC). We put quantitative constraints on the efficiencies of the third dredge-up and mass loss by requiring the models to reproduce the star counts, the luminosity functions and the colour distributions of the observed Oxygen-rich, Carbon-rich and extreme-AGB stars in the infrared pass-bands. Finally, we discuss the impact of the best-fitting prescriptions on the resulting chemical yields. These calibrated evolutionary tracks and isochrones will help to improve the predictive power of evolutionary population synthesis models and to interpret the upcoming data from the future observing facilities.
Characterisation of long-period variables in the Magellanic Clouds
Trabucchi, Michele
Asymptotic Giant Branch (AGB) stars often exhibit pulsation in modes of low radial order and periods between a few days and a few thousand days. The observed periods and variability amplitudes of such Long-Period Variables (LVPs) provide an additional constraint to be matched by stellar evolution and population models, as well as a powerful tool to infer stellar parameters. LPVs are known to follow several well defined period-luminosity (PL) relations at infrared wavelengths, representing promising distance indicators, especially in the case of the bright, large-amplitude Mira variables. In order to fully exploit the potential of LPVs in the study of stellar evolution and stellar populations, it is necessary to understand and characterise their observed variability features, both in relation to their photometric properties and the predictions from evolutionary and pulsation models. Of crucial importance is the correct identification of the actual pulsation modes involved in the observed PL sequences, a long debated topic. We present here a characterisation of the population of LPVs in the Magellanic Clouds. To do so, we make use of a large and improved grid of pulsation models, computed including updated opacity data for the specific composition of O- and C-rich stars. Pulsation models are combined with a population synthesis tool to simulate a population of evolved red giant variables. Models are compared with observational data from the OGLE-III Catalogue of Variable Stars, allowing us to provide a consistent interpretation of the observed PL sequences. We discuss some of the possibilities related to the use of pulsation models and observed pulsation properties to assist the calibration of stellar evolutionary models. Some of the issues involved in the theoretical modelling of pulsation in evolved red giants are also discussed.
On the origin of N in galaxies with galaxy evolution models
Vincenzo, Fiorenzo
Nitrogen is among the most abundant chemical elements in the cosmos and AGB stars are fundamental nucleosynthetic sources of this chemical element in galaxies. In my talk, I will show how the observed N/O versus O/H abundance pattern, both in extragalactic systems and in our own Galaxy, can be used to constrain the nucleosynthetic origin of N in the cosmos. The redshift evolution of chemical abundances in the ISM of galaxies can nowadays be studied both with "classical" chemical evolution models and with cosmological chemodynamical simulations; in my talk, I will review the results of my studies with both kind of galaxy evolution models focusing on the evolution of the N abundances in galaxies, by pointing out the most important critical issues in my studies.
O-rich LPVs in the X-shooter Spectral Library
Lancon, Ariane
The X-shooter Spectral Library contains, among others, the spectra of a few hundred luminous red stars, of which many are LPVs. The VLT/X-shooter spectra extend from the blue end of the stars' energy distributions to 2.5 micrometers in the near-infrared, and have a spectral resolving power of about 104. We will present a summary of the properties of these spectra, compare their statistical properties with those of previous collections, confront them with available models, and discuss their relevance for stellar population studies.