Abstracts of the Tuesday Pitch Session


Periodic variables as distance indicators in Gaia era

by Xiaodian Chen, Shu Wang, Licai Deng, et al.

Periodic variable stars, such as Cepheid, RR Lyrae and eclipsing binary, are important distance indicators. Historically, distance of some open clusters, globular clusters, Milky Way's dwarf galaxies and nearby galaxies are well determined by these distance indicators. However, both the rapid development of large, wide-field instruments and new insight of infrared facility, stimulate the researches of variables.

The role of variables on distance scale should be re-examined. With unprecedented Gaia DR2 parallax; the precision of W UMa-type contact binary period-luminosity relation is better than 6%. The huge number (0.1-0.2% of stars) makes it an important tracer of Milky Way's structure. For classical Cepheid; the number distributed in the Galactic disk increases rapidly this year. With the help of Gaia's proper motion and radial velocity; the spatial and kinematic stellar disk including arm; warp and flare; are revealed in high definition.


The ubiquity of unresolved triple stars discovered by the GALAH and Gaia

by Klemen Čotar, Tomaž Zwitter

The latest Gaia data release enables us to accurately identify stars that exhibit excess luminosity for its spectral type and distance. To uncover spectroscopically unresolved multiple systems in the scope of high-resolution spectroscopic survey GALAH; we first built observational stellar templates on the main sequence for different stellar types; where stellar spectra were combined based on their measured physical parameters.

The templates were used to find stars with near-identical spectra in the survey using a direct spectral comparison. For many of them; their absolute magnitude shows excess luminosity as they are placed above the main sequence were multiple stellar systems are expected to be. As they do not show any spectroscopic sign of multiplicity we combined photometric and spectroscopic observation to analyse their composition.

For this; we developed a data-driven methodology that can generate synthetic photometric signature and spectrum of a single star. By combining multiple such synthetic stars into unresolved binary or triple system; that was compared with the actual photometric and spectroscopic observations; we uncovered and characterized triple and binary stellar system candidates along the H-R diagram. The systems show barely any variation in their measured radial velocities; hinting to wide multiple systems with long orbital periods.


Formation of Barium stars constrained by Gaia parallaxes.

by Ana Escorza, Alain Jorissen

It is estimated that about half of the stars in the Galaxy have a binary companion which; at specific stages; can strongly affect their evolution. Binary interactions play a fundamental role in understanding stellar evolution and the chemical evolution of the Galaxy; however; many interaction processes are poorly understood. Our project focuses on chemically peculiar barium (Ba) stars; which are main-sequence or giant stars polluted with enriched matter through binary interaction by a former AGB companion (now a dim white dwarf; WD). The details of this interaction and the initial characteristics of binary systems that form barium stars are not known.

New observational constraints can lead to an improvement of theoretical models and to a better understanding of binary interaction physics in low- and intermediate-mass stars. We constrained orbital parameters for a sample of Ba stars at different evolutionary stages through long-term radial-velocity monitoring. Additionally; by combining stellar parameters; derived from high-resolution spectra and Gaia DR2 parallaxes; we located all of these Ba stars in the Hertzsprung-Russell diagram (HRD). Their masses were determined by comparing their positions on the HRD with STAREVOL evolutionary tracks. Combining these masses with the mass functions obtained from the orbital fitting; we derived the mass distribution for the unseen WD companions.

We obtained a wide range of WD masses; between 0.5 Msun and 1.1 Msun. Our results indicate that the WD companions of strongly enhanced Ba stars are more massive than those of mildly enhanced Ba stars. Including these constraints in binary evolutionary models can further our understanding of the initial conditions and interaction processes involved in the formation of Ba star systems.


Properties of Planetary Nebulae in Gaia DR2

by Iker Gonzalez, Minia Manteiga, Arturo Manchado, et al.

By carefully taking into account the recommended GoF for Gaia DR2 astrometry; we have selected a sample of about 300 PNe central stars for which precise distances can be obtained from the parallaxes measured by Gaia. For this sample we present an analysis of their astrometric properties; such as their galactic distribution and nebulae radii. We commented on the completness of DR2 measurements; and compare the nebular brightness with previous determinations based on models or theoretical constrains.


Cepheid variables in the Gaia Data Release 2 and the extragalactic distance scale

by Vincenzo Ripepi, Roberto Molinaro, Ilaria Musella, et al.

Classical Cepheids are the most important primary indicators for the extragalactic distance scale. Establishing the precise zero points of their Period-Luminosity and Period-Wesenheit (PL/PW) relations has profound consequences on the estimate of H0. Type II Cepheids are also important distance indicators and tracers of old stellar populations.

The recent Data Release 2 (DR2) of the Gaia Spacecraft includes photometry and parallaxes for thousands of classical and type II cepheids. Adopting a subsample of well studied objects, we use the Gaia parallaxes to derive new optical/Near Infra-Red PL/PW relations for the Galactic Cepheids. We use these information, coupled with proprietary data from the Vista Magellanic Cloud (VMC) survey, to discuss the zero point of the Gaia parallaxes and its impact on the extragalactic distance scale and the measurement of H0.


Determining the Luminosity of the Third Dredge-up via S-type stars: the promise of Gaia

by Shreeya Shetye

S stars are late-type giants whose spectra show distinctive molecular bands with as most noticeable characteristic the appearance of ZrO bands. They can be segregated in two classes: namely intrinsic and extrinsic. The 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 (TDU). The intrinsic S stars are the first ones on the AGB to have undergone a third dredge-up. Understanding the AGB nucleosynthesis and the TDU episodes in function of evolutionary mass and metallicity involves several challenges.

One such challenge is that on the AGB; the standard stellar evolution models produced TDUs for masses greater than 1.5Msun. However; there is a growing series of observations pointing at dredge-up occurring in lower mass stars (< 1.5 Msun). Here; for the first time thanks to a study of all intrinsic S stars in Gaia; we are able to test systematically the lower mass limit of TDU. The parameter study of intrinsic S stars along with their luminosity derived from the Gaia DR2 parallaxes has helped us marking the onset of the third dredge-up in the Hertzsprung-Russell (HR) diagram.

The accurate parallaxes from Gaia DR2 combined with the well-constrained Teff also helps further constraining the masses and surface gravities of these stars. The positions of the intrinsic S stars in the HR diagram show that the TDU occurs in stars with masses as low as 1 Msun and [Fe/H] in the range of -0.2 to -0.5. Comparison with new; dedicated stellar evolution and nucleosynthesis models show that with some adjustments they are able to reproduce the s-process enhancements of these objects. Moreover; this study is a part of a larger investigation on S stars with Gaia DR2 parallaxes.


On the iron ionization balance of cool stars: the role of accurate surface gravities from Gaia

by Maria Tsantaki, Nuno Santos, Sergio Sousa, et al.

High-resolution spectroscopic studies of G and K-type dwarf stars have revealed higher iron abundances derived from singly ionized species compared to neutral species; violating the ionization equilibrium under the assumption of local thermodynamic equilibrium. The "ionization balance problem" has troubled astronomers in several works of open clusters (Hyades; Pleiades; and M34) but also for field stars.

Several hypotheses have been proposed to explain the differences between FeII and FeI abundances but with no fully satisfactory answer yet. In this work; we investigate the overabundances of FeII lines reported in our previous work for a sample of 451 solar-type HARPS stars in the solar neighborhood. The spectroscopic surface gravities of this sample which emerge from the ionization balance; appear underestimated for the K-type stars. In order to understand this behavior; we search our FeII line list for unresolved blends and outliers.

For our sample we use a set of reference parameters (effective temperature and metallicity) and derive surface gravities using the Gaia DR2 parallaxes (trigonometric gravities); to calculate the FeI and FeII abundances. We exclude the FeII lines which produce overabundances above 0.10 dex. The derived surface gravities from the clean line list are now in agreement with the trigonometric. Moreover; the difference between FeI and FeII abundance does not show now a correlation with the effective temperature. Finally; we show that the ionization balance of Ti can provide better estimates of surface gravities than iron.

The more accurate gravities based on the Gaia parallaxes are used to improve the spectroscopic values. In this work; we use these gravities to provide a solution to the ionization balance problem observed in the atmospheres of cool dwarfs.


Stellar chemistry for Gaia from photometric narrow-band imaging

by Tomaž Zwitter

Strong metallic absorption lines can absorb a significant fraction of continuum flux in a stellar atmosphere; so its metallic content can be inferred from a contrast between fluxes in narrow-band images centred on and off these lines. This classical idea has been hampered by unknown values of stellar parameters which also influence line strengths. But Gaia DR2 results accurately place stars in the H-R diagram. Yet their chemistry remains unknown for all but very bright stars.

Position of a star in the H-R diagram depends on its mass and age; and to a smaller extent on its chemical composition. Even luminosities of main sequence stars increase with time; so effects of chemistry cannot be separated from age; and even less from stellar binarity. So position of field stars in the H-R diagram is not enough to infer their metallic content. Standard ugriz filters can be used to infer [Fe/H] values; as demonstrated by the SkyMapper survey.

But here we show that three off-the-shelf medium-width photometric filters can be used to determine both [Fe/H] and [α/Fe] with an accuracy of ~0.1 dex. We show that the corresponding photometric indices discriminate well between alpha enhancement and metallicity and that reddening is not a problem for all but the most obscured regions. This opens all objects down to V~17 to chemical population studies; a valuable goal for a photometric survey on a medium-sized telescope.