Division G - Abstracts
Hot Jupiters from Alternative High-eccentricity Migration Scenarios
Hamers, Adrian
One of the proposed formation origins of hot Jupiters (HJs) is high-eccentricity (high-e) migration. In high-e migration, the eccentricity of the orbit of a planet initially beyond the ice line is dynamically excited, leading to strong tidal dissipation occurring near periapsis and eventually resulting in a tight orbit. Previously studied secular variants of high-e migration involve an inclined and/or eccentric binary companion (star or massive planet). However, the predicted formation rates are too low compared to observations, by a factor of ~ 10. Therefore, alternative scenarios are needed. Recently, an alternative scenario was proposed by Lithwick & Wu involving secular chaos with three or more planets. This scenario can potentially produce many HJs because the initial conditions to achieve high eccentricities are less stringent compared to the case of an inclined binary companion. Until recently, studies of this scenario were limited to small phase space due to the prohibitive nature of direct N-body integrations. Here, we present a new secular method to study the long-term dynamics of hierarchical systems composed of nested orbits, with an arbitrary number of bodies and structure. We use this method to study the formation of HJs driven by secular chaos in multiplanet systems. Also, we consider scenarios in which HJs are formed through high-e migration in stellar triple systems, and discuss how secular evolution suggests the presence of unseen planetary companions to hot Jupiters in stellar binaries.
Helio and asteroseismic observational constraints on stellar properties accross the HRD
Chaplin, William
It is an exciting time to be involved in stellar astrophysics. A new generation of space- and ground-based telescopes are providing data of exquisite quality on unprecedented numbers of stars. In this review talk, I will provide a brief overview of how these data are opening up new possibilities for the detailed asteroseismic characterisation and study of stars across the Hertzsprung-Russell Diagram.
The asteroseismic revolution on Galactic scales
Chiappini, Cristina
In this review talk I will summarise how can asteroseismology of red giants contribute to the challenging task faced by Galactic Archeology of deciphering the assembly history of the Milky Way. Since the discovery of solar oscillation in red giants made by CoRoT in 2009, the field has evolved fast, and the pathway of how to produce high-resolution chrono-chemo-kinematical maps of the Galaxy seems now clear.It is now accepted that solar-like oscillating giants are excellent evolutionary clocks thanks to the availability of seismic constraints on their mass and to the tight age–initial mass relation they adhere to. Their larger luminosities (in comparison with sub-giants) together with their large age-baseline make of them perfect galactic mappers to larger distances than what will be possible with Gaia plus spectroscopy only.The first steps taken in this direction by combining spectroscopic surveys such as APOGEE, RAVE , and Gaia-ESO with seismic information from Kepler, K2 and CoRoT will be briefly summarised, as well as the current main caveats and difficulties. Key outstanding and still open questions in Galactic Archaeology will be summarised with the purpose to illustrate the need for precise ages for large samples of stars covering large portions of the Galaxy.I will conclude by showing that when these questions are turned into asteroseismic requirements we can show these to be within the capabilities of the current PLATO instrument design, provided the observations are sufficiently long to produce data of sufficient quality to allow for age estimates of distant targets to a 10% precision level. We therefore strongly endorse PLATO’s current design and proposed observational strategy, and conclude that PLATO can be turn into a legacy mission for Galactic archaeology.
Some issues in stellar modelling
Basu, Sarbani
Stellar models are only as good as the inputs used to construct them. These may be microphysics inputs, such as radiative opacities and equation of state, or macrophysics effects such as diffusion and gravitational settling, convective overshoot, and rotation-induced mixing. I shall review how uncertainties in some key inputs to stellar models affect our interpretation of stellar data, and what needs to be done to make stellar models more robust.
Synergy between asteroseismology and exoplanet science: an outlook
Campante, Tiago
Over the past decade, space-based asteroseismology has played an important role in the characterization of exoplanet-host stars and their planetary systems. The future looks even brigther, with space missions such as TESS, CHEOPS and PLATO ready to take on this legacy. In this talk, I will start by reviewing current key synergies between asteroseismology and exoplanetary science, such as the precise determination of radii and ages of host stars, the measurement of spin-orbit alignment, and the determination of orbital eccentricity via asterodensity profiling. I will conclude with an outlook on future synergies (e.g., the precise characterization of super-Earths/Neptunes orbiting solar-type stars and the prospect of conducting a populational study of giant planets around evolved stars) and further provide an overview of the asteroseimsic yield of exoplanet-host stars expected for the upcoming TESS mission.
Mass and Age of Red Giant Branch Stars Observed with LAMOST and Kepler
wu, yaqian
Obtaining accurate and precise masses and ages for large numbers of giant stars is of great importance for unraveling the assemblage history of the Galaxy. In this paper, we estimate masses and ages of 6940 red giant branch (RGB) stars with asteroseismic parameters deduced from \emph{Kepler} photometry and stellar atmospheric parameters derived from LAMOST spectra. The typical uncertainties of mass is a few per cent, and that of age is $\sim$\,20 per cent. The sample stars reveal two separate sequences in the age -- [$\alpha$/Fe] relation -- a high--$\alpha$ sequence with stars older than $\sim$\,8\,Gyr and a low--$\alpha$ sequence composed of stars with ages ranging from younger than 1\,Gyr to older than 11\,Gyr. We further investigate the feasibility of deducing ages and masses directly from LAMOST spectra with a machine learning method based on kernel based principal component analysis, taking a sub-sample of these RGB stars as a training data set. We demonstrate that ages thus derived achieve an accuracy of $\sim$\,24 per cent. We also explored the feasibility of estimating ages and masses based on the spectroscopically measured carbon and nitrogen abundances. The results are quite satisfactory and significantly improved compared to the previous studies.
Spatially resolved spectroscopy across stellar surfaces
Dravins, Dainis
Center-to-limb changes of spectral line profiles could in the past be studied only on the spatially resolved Sun but are now becoming accessible for also other stars. During exoplanet transits, successive stellar surface portions become hidden and differential spectroscopy between various transit phases provide spectra of small surface segments temporarily hidden behind the planet [1]. Such retrievals of spatially resolved high-resolution photospheric line profiles have now been achieved along the exoplanet transit chords across HD209458 (G0 V) and HD189733A (K1 V), using data from the ESO UVES and HARPS spectrometers [2,3]. Not being subject to rotational broadening, spatially resolved profiles are narrower and deeper than those of integrated starlight while their gradual shifts in wavelength reflect stellar rotation at the latitude of exoplanet transit. Synthetic spectral lines from 3-D hydrodynamic models predict various center-to-limb signatures that differ among stars of different temperature, such as a gradually increasing linewidth toward the stellar limb, caused by horizontal velocities in stellar granulation being greater than vertical ones. Since even giant planets cover only a tiny fraction of any solar-type star, the directly observable signal is small and extremely precise observations are required. To reach adequate signal-to-noise ratios, requires averaging over numerous lines with similar parameters. The method will likely be applicable to several more stars in the near future, as new targets are being found by ongoing searches for bright host stars with transiting large planets. [1] Dravins, D., Ludwig, H.-G., Dahlén, E., & Pazira, H. 2017a, Astron. Astrophys., 605, A90 [2] Dravins, D., Ludwig, H.-G., Dahlén, E., & Pazira, H. 2017b, Astron. Astrophys., 605, A91 [3] Dravins, D., Gustavsson, M. & Ludwig, H.-G. 2018, Astron. Astrophys., to be submitted
Oscillation mode variability in pulsating hot B subdwarfs and white dwarfs from Kepler photometry
Zong, Weikai
The Kepler satellite provides unprecedented and uninterrupted high-quality photometric data with a time baseline of about 4 yr collected on pulsating stars, which is the unique opportunity to characterize the long-term behaviours of oscillation modes. A mode modulating in amplitude and frequency can be independently inferred by its fine structure in the Fourier space, detected by the sliding Lomb-Scargle periodogram, and measured by prewhitening the entire light curves parts by parts. We apply these techniques to the evolved compact stars KIC 3527751, KIC 08626021 and KIC 10139564 and find that many rotational multiplets whose components show clear amplitude and/or frequency variations. These modulations can be periodic, irregular and stable over the Kepler observations, which are the first signatures of nonlinear interactions due to the resonant mode coupling theory. Our results suggest that oscillation modes with diverse variations should be a common phenomenon in pulsating sdB and white dwarf stars. This resonate an idea that the closed peaks need to be seriously examined for mode identification, which is a key input parameter of seismic modelling. These various modulation patterns motivate more precise stellar oscillation theory to be developed. It also raise a warning to any long-term project aiming at measuring the rate of period change of pulsations due to stellar evolution or discovering stellar (planetary) companions around pulsating targets. These phenomena can also be thoroughly examined in many types of pulsating stars over the entire HR diagram from the photometry of the upcoming TESS mission.
The Kepler Pixel Project
Szabo, Robert
Kepler photometry is so precise that new ways can be developed to harvest the great wealth of quasi-continuous data that has never been accessible from the ground. We have initiated a new project that we dub The Kepler Pixel Project in order to explore approaches and to discover new pulsating stars and other time-variable objects.In this endevour we examine individual pixels of the original Kepler mission to find interesting objects around the main Kepler targets. Specifically we launched a project to find background, faint RR Lyrae stars that are missing from the Kepler sample. In this talk the first results of The Kepler Pixel Project are presented. We will discuss the project, the search algorithms, new findings, as well as the properties of the enlarged Kepler RR Lyrae sample.We will also show other results and examples (asteroseismology, new eclipsing binaries, transiting exoplanets, flares, outbursts) to demonstrate the potential and future avenues of the project.
Massive stars in the X-ray light
Oskinova, Lidia
Massive stars on nearly all evolutionary stages emit X-rays. Recent advancesin observations challenge previously accepted concepts of mechanismsresponsible for this X-ray emission. A possible new paradigm is emerging,considering the X-ray radiation from massive stars as evidence forconnections between the stellar photosphere and the wind. I will brieflyreview what we can learn from the most recent and spectacular observationsof massive stars, such as X-ray pulsations in B-type stars, X-ray aurorae onmagnetic stars, and X-ray variability in O and WR stars with their strongwinds. I will discuss some outstanding questions in massive starastrophysics that could be answered by observations with future X-raytelescopes, and highlight the required synergies with the next generation ofground-based telescopes.
Spectroscopic Temperatures, Gravities, and Metallicities for Kepler Giants
Peterson, Ruth
Motivated by the need for reliable stellar parrameters in applying asteroseimic observational constraints to derive fundamental stellar parameters, we report the results of a pilot study that uses Keck HIRES spectra exclusively to determine temperatures, gravities, and metallicities for giants of solar-metallicity and higher that lie within the Kepler field. Our approach is independent of the photometry, reddening, and isochrones generally invoked in making such determinations, as these have proven somewhat problematical in asteroseismological studies (e.g. Corsaro et al. 2017, A&A, 605, A3).Our primary focus is on NGC 6791, the metal-rich old open cluster that serves as a benchmark for metal-rich systems near and far. Based on Keck spectra for about twenty stars, we determine preliminary radial velocities as a first indicator of membership. We then determine temperature (from Fe I lines over a wide range of excitation), gravity (from Fe II lines and the wings of strong lines), and metallicity (from weak Fe I and Fe II lines), by running spectral synthesis for every star, to match as much of each HIRES spectrum as is feasible.We begin with the relatively weak-lined warm stars near the cluster turnoff, and continue upwards in luminosity to reach the very strong-lined, cool stars of the giant branch. With the help of brighter analogs of these stars outside the Kepler field, and our decades-long effort to improve atomic-line gf values by intercomparing stars across a range of spectral types, we are able to significantly reduce the uncertainties of these critical determinations.
Revisiting young alpha-enhanced stars in CoRoT center and anti-center fields
Valentini, Marica
In 2015 two studies pointed out for the first time the existence of a young stellar population enriched in alpha-elements (Chiappini et al. 2015 and Martig et al 2015). By combining asteroseismology and spectroscopy, it was possible to derive masses, and hence ages, of a big sample of red giants observed by Kepler and CoRoT satellites and belonging to the APOGEE spectroscopic survey. Among this sample, a group of stars standed out, being massive, and hence young, but possessing an alpha-enrichment tipical of older populations. Two scenarios have been proposed for explaining this objects: a. they are blue-stragglers, stars that rejuvenated via mass accretion from a companion; b. these objects formed close to the end of the Galactic bar, near corotation, a region where gas might have been kept inert for longer times than in other regions.In this work we consider stars belonging to CoRoT fields LRc01 and LRa01, the former pointing at the inner part of the Galaxy and the latter pointing at the opposite region. The sample is located in the Galactic disc and spans distances from 4 Kpc to 12 Kpc from the Galactic centre. Stars were observed by APOGEE and Gaia-ESO surveys, and have precise atmospheric parameters and chemical abundances.We derived masses and ages of the stars using an updated version of the Bayesian code used in Chiappini et al. 2015. We also investigated the impact in the age and mass determination of temperature/metallicity shifts, different mass-loss approaches, alpha-enrichment and corrections on the seismic scaling relations. Even with this new approach, and using data coming from two different surveys, we identified a population of young alpha-enhanced stars.
NLTE abundances of C, O, Ca, Ti, and Fe in BAF-type stars
Sitnova, Tatyana
A knowledge of accurate chemical abundances of BAF-type stars is important for understanding stellar physics and the galactic matter at modern epoch. We treat accurate methods of abundance determination based on the non-local thermodynamic equilibrium (NLTE) line formation for five chemical elements, namely carbon, oxygen, calcium, titanium, and iron. We apply comprehensive model atoms of C I-II, O I, Ca I-II, Ti I-II, and Fe I-II described in our previous studies. It is worth noting that we are the first who have performed the NLTE calculations with comprehensive model atoms for Ca I-II, Ti I-II, and Fe I-II in the Teff range from 7000 K to 13000 K. We determine the NLTE abundances of C, O, Ca, Ti, and Fe in ten BAF-type stars with well determined atmospheric parameters using high-resolution and high signal-to-noise ratio spectral observations in wide wavelength range, from UV to IR. We show that, for each chemical species in each star, NLTE leads to consistent abundances from different lines. Our NLTE methods well reproduce emission lines of C I, Ca II, and Fe II detected in the near IR spectrum of late B type subgiant star HD160762, using classical plane-parallel model atmosphere. In BAF-type stars, all the investigated elements except O are observed in lines of the two ionisation stages, providing an opportunity to determine atmospheric parameters from the ionisation equilibrium method. We perform the NLTE calculations for these elements and provide departure coefficients for atomic levels in a grid of model atmospheres with 7000 K < Teff < 13000 K, 3.2 < log g < 5.0, -0.5 < [Fe/H] < 0.5. The grid of departure coefficients was implemented in the SME code for automatic determination of atmospheric parameters and abundances.
Testing the Stellar Rotation vs. Age Paradigm Using Wide Binaries in the Kepler K2 Fields
Oswalt, Terry
Essential to understanding the history of the Galaxy’s stellar populations, ages are among the most difficult to measure properties of stars. Accurate stellar ages would provide key leverage on problems ranging from the habitability of exoplanets to the Galaxy’s chemical evolution and age. Gyrochronology, the empirical relation between rotation and age, is believed to be among the best current methods of stellar age determination. Using several hundred wide non-interacting binaries identified in the Kepler mission “K2” extended mission fields we are testing this paradigm. Such wide pairs afford a unique opportunity to unravel and calibrate the effects of mass, rotation, activity and age because components of a given binary should have rotation and activity levels consistent with their masses and a common age. We present preliminary results for several dozen wide pairs in which both components exhibit detectable rotational modulation.
Rings around B[e] Supergiants
Maravelias, Grigoris
Massive stars affect strongly the insterstellar medium through their intense stellar winds and their rich chemically processed material as they evolve. In specific short-lived transition phases (e.g. B[e] Supergiants, Luminous Blue Variables, Yellow Hypergiants) the mass-loss becomes more enhanced and usually eruptive. This leads to the formation of a complex circumstellar environment, which is not always well understood. To improve our knowledge on these phases we examined the structures found around the B[e] supergiants for a sample of Galactic and Magellanic Cloud sources. Using high-resolution optical and near-infrared spectra, we examine a set of key emission features ([OI], [CaII], CO bandheads) to trace their physical conditions and kinematics in their formation regions. Assuming Keplerian rotation of the circumstellar material we find that each B[e] Supergiant is surrounded by a unique distribution of single and/or multiple equatorial rings. Moroever, we find that these structures seem to be more stable and long-lived around single stars rather than binaries that show significant spectroscopic and photometric variability.
Detecting Galactic halo planetary nebulae and symbiotic stars in multi-band photometric surveys
Gutiérrez Soto, Luis Angel
About 3,000 planetary nebulae (PNe) and 251 symbiotic stars (SySts) are known in our galaxy. From them, only 14 PNe and around 5% of SySts are located in the Galactic halo or in high latitudes.In this contribution, we present the preliminary results of a research to identify halo PNe and SySts in the direction of halo, by using the Javalambre-Physics of the Accelerating Universe Astrophysical Survey (J-PAS), the Javalambre-Photometric Local Universe Survey (J-PLUS) and the Southern Photometric Local Universe Survey (S-PLUS). The main advantage of these surveys is their great combination of narrow- and broad-band filters from 3,000-10,000 Å, in total 56, 12, and 12, respectively. The optical spectra of several types of sources as well as a grid of photoionization models, convolved to the photometric systems of the three surveys, are used to generate combinations of colour-colour diagrams that discriminate halo PNe and SySts from other emission line objects (star-forming galaxies, QSOs, cataclysmic variables, extragalactic H II regions, young stellar objects (YSOs), Be stars, among others). The J-PLUS science verification data of two known halo PNe (H 1-4 and PNG 135.9+55.9) are found to located close to the region of halo PNe. New halo PNe and SySts candidates, selected in the J-PLUS Early Data Release, will be discussed in this contribution.
New Perspectives on Classical Pulsators
Smolec, Radoslaw
In recent years, we have witnessed many discoveries related to classical pulsators, Cepheids and RR Lyrae (RRL) stars, accompanied with progress in our understanding of these invaluable astrophysical tools. These discoveries are possible not only thanks to the precise observations of space missions, but also thanks to long-lasting regular monitoring of tens of thousands of classical pulsators by ground-based surveys. I will present the recent discoveries done by Warsaw group using the data of the OGLE project. Five new groups of double-periodic RRL stars were recently discovered. These include not only radial mode pulsators, such as anomalous RRd stars, but also three groups in which non-radial pulsation is excited. For one of them, identification of additional periodicities with non-radial modes of moderate degrees will allow for asteroseismic inferences. Still, the mode content and mechanism behind multiperiodic pulsation remains puzzling in most of the new groups. Analysis of classical Cepheids photometry in search for additional modes is ongoing. I will present detailed analysis of first overtone Cepheids with moderate degree non-radial modes that form three sequences in the Petersen diagram, as well as of stars with close non-radial modes. Finally, quasi-periodic modulation of pulsation, so-called Blazhko effect, was discovered in all types of classical pulsators. Modulations are a common feature of fundamental mode RRL stars. They are also common in some of the new groups of double-periodic variables. I will focus however on a periodic modulation of pulsation in Cepheids, which is a new phenomenon. A numerous sample of modulated stars was found among fundamental mode classical Cepheids, the most useful standard candles. Modulation was also discovered for the first time in tens of type-II Cepheids. These modulations are most likely of resonant origin. No doubt classical pulsators are much more complex and puzzling stars than just a few years ago.
Nova Sagittarius V5668 - the first three years
McLoughlin, Dominic
Nova Sagittarius V5668 is a nova that detonated in March 2015 on which we have a densely time-sampled set of observations, from before it reached maximum light until the present day. These spectra are from the Global Jet Watch observatories and span over 1000 days, capturing in detail the very distinct phases of a nova eruption. Our investigations include a prominent dust dip which commenced in the first 100 days, and analysis of many significant changes in the line profiles arising from geometry of the obfuscating dust and the inner binary. Rapid and dramatic changes are observed in outflows of oxygen especially. I will discuss heavy elements detected shortly after detonation and the implications for the history of the progenitor stars. I will present the different dynamical components revealed by the evolving H-alpha complex.
Detailed chemical abundances of M dwarf planet host stars from APOGEE observations
Souto, Diogo
In this work, we present the first results of a detailed chemical study of M-dwarf star where we demonstrate that it is possible to obtain elemental abundances of up to thirteen elements (C, O, Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn and Fe) from an LTE analysis of high-resolution near-infrared spectra, between 1.5 - 1.7 microns obtained by the APOGEE survey. Two M-dwarfs with exoplanets detected by the Kepler mission, Kepler-138 and Kepler-186 were analyzed. Our results indicate that both stars have near-solar metallicities, [Fe/H] = -0.09 and -0.08 dex, for Kepler-138 and Kepler-186, respectively. One interesting result obtained was that the star Kepler-186 exhibits an enrichment of silicon ([Si/Fe] = +0.18), and this element is important in controlling the internal structure of rocky planets.
Asteroseismic constraints on stellar opacities
Walczak, Przemyslaw
One of the most interesting and still unresolved puzzle of the massive star interiors is the shape of the opacity profile. The opacity coefficient determines the energy transport and influences pulsational properties. Due to the latter we can use asteroseismic observables as diagnostic tools for the opacity inference.The excellent targets for such an analysis are the hybrid pulsators of ß Cep/SPB typewhich pulsate in both pressure (p) and gravity (g) modes. The simultaneous excitation of p and g modes offersprobing stellar regions sensitive to various physical processes. A lot of new members of this group were discovered thanks to space-borne observations from the projects like CoRoT, Kepler and BRITE.We show that models computed with widely-used opacity data (OPAL, OP, OPLIB) cannot explain the pulsations of these stars, in particular the instability of high-order g modes. Applying the complex seismic modelling to a few hybrid pulsators, we found that a significant modification of the mean opacity profile is indispensable to explain all pulsational properties.
Variability Phenomena in the Gaia second Data Release (DR2)
Holl, Berry
Gaia is an all-sky multi-epoch ESA mission observing over a billion sources during its five-year nominal mission duration. Besides astrometric and spectroscopic measurements, it collects photometric time series data in three pass-bands that allow us to detect and describe variability phenomena in an unprecedented way. The Gaia second data release published in April 2018 is based on 22 months of observations and contains classified and characterised light curves for more than 500,000 variable source candidates consisting of: RR Lyrae stars, Cepheids, Long Period Variables, Delta Scuti/SX Phoenicis and BY Draconis stars. A list of various short time-scale variability phenomena is also included. We present an overview of this rich data set, the diverse properties of these variable sources, and their distribution in the colour-magnitude diagram. Though this is still a relatively small fraction of the variable stars that Gaia detects, these catalogues are already among the largest ones ever published.
Asteroseismology versus Gaia: How Accurate are Scaling Relations for Solar-like Oscillators?
Huber, Daniel
Gaia DR2 will include high-precision parallaxes for nearly all ~15,000 Kepler targets showing solar-like oscillations, providing an unprecedented sample to test the accuracy of fundamental stellar parameters derived from asteroseismology over a wide range of evolutionary states. I will present results of comparing and calibrating asteroseismic scaling relations using Gaia DR1 (TGAS) and DR2 parallaxes, including over a dozen oscillating red giants with near model-independent radii measured by combining parallaxes with angular diameters from long-baseline interferometry using the CHARA Array. I will furthermore use APOGEE spectroscopy to investigate deviations from scaling relations as a function of metallicity and evolutionary state, and discuss the implications of such deviations for the characterization of exoplanet host stars and galactic archeology.
Milky Way structure based on thousands of Cepheids and RR Lyrae stars from the OGLE survey
Pietrukowicz, Pawel
Classical Cepheids and RR Lyrae-type variable stars are widely-used tracers of young (<300 Myr) and old (>10 Gyr) stellar populations, respectively. These stars also serve as distance indicators allowing for Galactic structure studies. Confident detection of pulsating variables requires precise and relatively frequent observations over several years. Recently, the OGLE survey has discovered nearly one thousand new genuine classical Cepheids and two thousands RR Lyrae stars along the Galactic plane in the III and IV quadrants. I will present the view of the spiral structure drawn from known Cepheids and the picture of the Milky Way old population that emerges from the set of known RR Lyrae stars.
Pulsations as Probes in Hydrogen-Deficient Atmospheres
Jeffery, Simon
The extreme helium stars V652 Her and BX Cir exhibit large amplitude radial pulsations with periods of 0.1 d. High spectral and temporal resolution observations have been made at both optical and ultraviolet wavelengths to explore the surface motion, demonstrating differential motion within the visible photospheres and, in particular, the passage of a shock wave through the photosphere at minimum radius. These observations are being used to provide strong tests of non-linear pulsation theory. We will present new Hubble Space Telescope spectroscopy of V652 Her, which was obtained to probe photospheric layers over a range of optical depths not visible at optical wavelengths. We will also present recent SALT and K2 observations of related extreme helium stars and discuss the implications for opacity driven pulsations in early-type extreme helium stars.
Deciphering the traces of mixing processes in interiors of massive g-mode pulsators
Szewczuk, Wojciech
Our understanding of massive star physics is still deficient.This is mainly due to imperfect description of mixing processes which strongly affectthe structure and evolution. The processes like overshooting from convective regionsor rotational-induced mixing are described by free parameters which are poorly calibrated.Seismic studies of Slowly Pulsating B-type stars (SPB) gives a hope for progress in this issue.Inside these pulsators the buoyancy waves (called g modes) are excitedand they are very sensitive to the conditions prevailing in the deep interior.The traces of mixing processes are saved in particular in the period spacingwhich was identified in many B-type stars observed by Kepler.Using MESA evolutionary code and taking into account the effects of rotationin nonadiabatic pulsational computation, we try to get constraints on mixing parameters.
Stability of hierarchical stars in globular star clusters
Hypki, Arkadiusz
We present the first results concerning the stability of hierarchical systems of stars (triples and higher) in globular clusters using a broad range of numerical simulations done with the MOCCA code (http://moccacode.net).Globular clusters (GCs) are unique laboratories for studying various types of stars or stellar systems. There are numerous papers about the role which single and binary stars play in the stellar and dynamical evolution of GCs. However, the formation and dynamical evolution of higher hierarchies are poorly covered. The interest in hierarchical systems has increased in the recent years significantly as it was realized how important they can be for the formation and evolution of exotic objects, like close binary systems.The MOCCA code is currently one of the most advanced codes which is able to follow the full stellar and dynamical evolution of the real size GCs. Recently, MOCCA received procedures which allow to form and then follow the dynamical evolution of hierarchical systems. This code is orders of magnitude faster than N-body codes which allows to perform a broad range of numerical simulations for various initial conditions. This, in turn, allows to perform statistical analysis of the stability of hierarchical systems for many different GCs.
The Global Supernova Project
Howell, Andrew
I'll present the latest results from the Global Supernova Project, a 150+ person collaboration using Las Cumbres Observatory (LCO) to study 500 supernoave over 3 years. LCO is a robotic network of 21 0.4, 1, and 2m telescopes spread around the globe to allow instantaneous and extended access to the night sky. Recent results include the discovery of blue bump from the first hours after a Type Ia supernova, which we interpret as the shock of a supernova hitting a nondegenerate companion star. We have also used the cooling after shock breakout to measure the progenitors of core-collapse supernovae. I'll also prevent several lines of evidence indicating that circumstellar material is ubiquitous in core-collapse supernovae, even in cases where no narrow lines from interaction are seen.
Gravitational and Electromagnetic signal from ultra-compact binaries
Wang, Di
The event GW170817 with both gravitational and electromagnetic signal marks the beginning of the multi-messenger astronomy. For an ultra compact pulsar binary, e.g., a neutron star and white dwarf binary system, effects like tide and spin induced quadrupole can influence both the pulsar arrival time and waveform of gravitational radiation. Theoretical and numerical analysis on such effects allow us to extract useful information on both neutron star and white dwarf, which are of interest to space detectors of gravitational wave.
Variability Phenomena in the Gaia second Data Release
Rimoldini, Lorenzo
Gaia is an all-sky multi-epoch ESA mission observing over a billion sources during its five-year nominal mission duration. Besides astrometric and spectroscopic measurements, it collects photometric time series data in three pass-bands that allow us to detect and describe variability phenomena in an unprecedented way. The Gaia second data release published in April 2018 is based on 22 months of observations and contains classified and characterised light curves for more than 500,000 variable source candidates consisting of: RR Lyrae stars, Cepheids, Long Period Variables, Delta Scuti/SX Phoenicis and BY Draconis stars. A list of various short time-scale variability phenomena is also included. We present an overview of this rich data set, the diverse properties of these variable sources, and their distribution in the colour-magnitude diagram. Though this is still a relatively small fraction of the variable stars that Gaia detects, these catalogues are already among the largest ones ever published.