Abstracts of the Tuesday Programme


Towards a Complete Census of GAIA CSPNe

by Nicholas Chornay, Nicholas Walton

Planetary Nebulae (PN) are a brief stage through which low and intermediate mass stars pass towards the end of their evolution; between red giant and white dwarf. They are excellent probes of the chemo-kinematical structure of the Milky Way. Gaia is optimised for the detection of point sources; and is in general not sensitive to extended objects (with sizes >0.5 arcsec).

However; Gaia is able to resolve local point-like maxima within extended objects - e.g. in commissioning observations of PN NGC 6543; this complex nebula was decomposed by Gaia into thousands of individual sources. Thus Gaia can detect not only resolved PN central stars; but also; even in the event that the central star is faint or obscured by nebula emission; features of the nebula itself. We present initial results from Gaia Data Release 2; determining the parallaxes to a sample of thousands of known galactic PN.

Our technique employs a semi-automated selection process using location and colour information to match the PN central stars in the input catalogue with Gaia sources. Starting from a set of high-confidence cross matches; we generalise these to optimally match more extended or confused Gaia sources. Additionally; by exploiting the astrometric and colour properties such as error and flux excess; we recover and match with their Gaia sources a set of recently discovered PN from ground based imagery. We note our results in discovering new PN from the Gaia sources based on these properties.

We will extend our classification technique to incorporate machine learning to automate the discovery and matching of emission line PN; and also plan to deploy it to the medical domain; specifically in cell classification from high resolution protein and gene species annotated breast cancer imagery.


Invited Talk: Discovering multiply-imaged quasars with Gaia

by Ludovic Delchambre, Alberto Krone-Martins, Christine Ducourant, et al.


Gaia was mainly designed for the study of stars in our Galaxy. However, by continuously scanning the entire sky, it also provides informations on all kind of objects that fall in its fields of view, including extragalactic objects. Amongst these extragalactic sources, quasars stand out to be some of the most peculiar objects that Gaia observe. Beside the fact that they play a crucial role in fixing the Gaia Celestial Reference Frame, they also have their intrinsic interest in cosmology.

One of their important application occurs when a massive galaxy stands along the line of sight joining the quasar and the observer as, in this case, multiple images of this quasar may form. The time delays measurement between these lensed images then provide a reliable estimation of H0 that is independent of the commonly used cosmic distance ladder.

We describe here how the precise relative positions and magnitudes, as provided by Gaia, can be used in order to probe the lensing nature of clusters of objects coming from the Gaia DR2. We present some newly discovered lenses that were spectroscopically confirmed in recent follow-up observations. Finally, we see how the upcoming Gaia data releases will further allow to increase the number of discovered lenses by one order of magnitude as compared to the number of those that are presently known.


Gaia parallaxes versus updated pulsation model predictions

by Giulia De Somma, Marcella Marconi, Roberto Molinaro, et al.

The Hubble constant tension between the value derived by Riess et al. (2016;2018) on the basis of Classical Cepheids distances and the Cosmic Microwave Background results is one of the most debated issues in current astrophysical literature. On the basis of an updated and extended set of nonlinear convective pulsation models; we are theoretically investigating the systematic effects on the Cepheid based extragalactic distance scale and the resulting Hubble constant. The predicted light curves and pulsation relations for both Galactic and Magellanic Cepheids; based on these new model sets; are tested on Gaia Data Release 2 (DR2) results.

We find evidence of a Gaia DR2 parallax offset; consistent with independent results in the literature; but a general good agreement with the distances predicted by our theoretical pulsation scenario. We discuss the implications of our results and future perspectives; while waiting for Gaia Data Release 3.


Uncovering Hidden Quasars behind the Galactic Plane with ALLWISE and Gaia

by Yuming Fu, Xue-Bing Wu, Qian Yang, et al.

The search for Quasars behind the Galactic Plane (GPQs) has always been difficult not only owing to severe extinction and reddening of the Galaxy; but also contamination and obscuration by the crowded stars. New GPQ sample will help enlarge the whole population of known quasars; improve astrometric precision by serving as reference standards; and may probe gas contents and structure of the Milky Way.

Recently; we have developed new selection methods using statistical and machine learning techniques with ALLWISE and Gaia data. Both Gaia astrometry and photometry are found to be exceedingly effective in removing stellar contaminations. And a preliminary GPQ candidates table with over 35 000 sources at |b| <= 10 deg has been obtained; in which tens of the candidates have been confirmed through spectroscopic observations. This project will also show how GPQ sample can help study the systematics in Gaia astrometry near the Galactic plane.


Gaia Astrometry and Gravitational Waves

by Robin Geyer, Sergei Klioner, Uwe Lammers


A gravitational wave (GW) passing through an observer shifts the observed positions of all celestial sources periodically and coherently all over the sky. Although for expected strains of GWs this shift is very small, the effect may still be measurable under certain circumstances using high-precision astrometry of Gaia.

We argue that Gaia astrometry is sensitive for GWs with periods of days to years, covering a frequency range in which other detectors are partially blind. The standard astrometric solution of Gaia does not model GW signals and this can lead to a specific systematic error if such GW signals are present in reality. The effects of an unmodeled GW signal on the standard astrometric solution will be clarified in this contribution.

A method to detect GW signals from the residuals of the astrometric solution will also be presented. Using this method, it is demonstrated, for simulated Gaia-like data, that a GW signal can be detected without any prior knowledge of GW parameters. The sensitivity of Gaia observations to GWs depending on GW's frequency and direction will be also discussed.


Variability results in DR2: what; where and how to use them.

by Berry Holl, Marc Audard, Krzysztof Nienartowicz, et al.

The Gaia Data Release 2 published the time series; classification and photometric properties of 550 737 variable sources of types: RR Lyrae stars; Cepheids; Long Period Variables; stars with rotation modulation; δ Scuti and SX Phoenicis stars; and short-timescale variables. We estimate that about half of them are newly identified variables. In this presentation we present an overview of the properties of this data; where to find them in the archive tables; and how to efficiently access and use them. We end with an outlook of what can be expected in DR3.


A white dwarf catalogue from Gaia-DR2 and the Virtual Observatory

by Fran Jiménez-Esteban

We present a catalogue of 73,221 white dwarf candidates extracted from the astrometric and photometric data of the recently published Gaia-DR2 catalogue. White dwarfs were selected from the Gaia Hertzsprung-Russell diagram with the aid of the most updated population synthesis simulator.

Our analysis shows that Gaia has virtually identified all white dwarfs within 100 pc from the Sun. Hence; our sub-population of 8555 white dwarfs within this distance limit and the colour range considered; -0.52<(G_BP-G_RP)<0.80; is the largest and most complete volume-limited sample of such objects to date. From this sub-sample; we identified 8,343 CO-core and 212 ONe-core white dwarf candidates and derived a white dwarf space density of 4.9± 0.4 x 10^{-3} pc^{-3}. A bifurcation in the Hertzsprung-Russell diagram for these sources; which our models do not predict; is clearly visible.

We used the Virtual Observatory SED Analyzer tool to derive effective temperatures and luminosities for our sources by fitting their spectral energy distributions; that we built from the ultraviolet to the near-infrared using publicly available photometry through the Virtual Observatory. From these parameters; we derived the white dwarf radii. Interpolating the radii and effective temperatures in hydrogen-rich white dwarf cooling sequences; we derived the surface gravities and masses.

The Gaia 100 pc white dwarf population is clearly dominated by cool (~8,000 K) objects and reveals a significant population of massive (M~0.8 Solar Mass) white dwarfs; of which no more than 30-40 per cent can be attributed to hydrogen-deficient atmospheres; and whose origin remains uncertain.


New light on the Gaia DR2 parallax zero-point: influence of the asteroseismic approach; in and beyond the Kepler field

by Saniya Khan, Andrea Miglio, Benoit Mosser, et al.

The importance of studying the Gaia DR2 parallax zero-point by external means was underlined by Lindegren et al. (2018); and initiated by several works making use of Cepheids; eclipsing binaries; and asteroseismology. Despite a very efficient elimination of basic-angle variations; a small fluctuation remains and shows up as a small offset in the Gaia DR2 parallaxes.

By combining astrometric; asteroseismic; spectroscopic; and photometric constraints; we undertake a new analysis of the Gaia parallax offset for nearly 3000 red-giant branch (RGB) and 2200 red clump (RC) stars observed by Kepler; as well as about 500 and 700 red giants (both RGB and RC) selected by the K2 Galactic Archaeology Program in campaigns 3 and 6. Engaging into a thorough comparison of the astrometric and asteroseismic parallaxes; we are able to highlight the influence of the asteroseismic method; and measure parallax offsets in the Kepler field that are compatible with independent estimates from literature and open clusters.

Moreover; adding the K2 fields to our investigation allows us to retrieve a clear illustration of the positional dependence of the zero-point; in general agreement with the information provided by quasars. Lastly; we initiate a two-step methodology to make progress in the simultaneous calibration of the asteroseismic scaling relations and of the Gaia DR2 parallax offset; which will greatly benefit from the gain in precision with the third Data Release of Gaia.


Properties of the Gaia-ICRF3 radio-optical reference frame

by Valeri Makarov, Ciprian Berghea

The radio-optical reference frame tie is based on a few thousand radio-loud quasars listed in the ICRF3 with sufficiently bright optical counterparts to be observed by Gaia. The positions of these sources do not always match within reasonably expected errors; with about 1/5 of the pre-selected sources showing large offsets. We collect a sample of some 2100 quasars; which are not perturbed and appear to be bona fide point-like sources.

With these vetted sources; we perform analysis of the reference frame differences using the vector spherical harmonic decomposition and singular value decomposition. The estimated parameters quantify possible systematic errors; as well as important physical phenomena such as the acceleration of the Solar barycenter. The eigenvalues indicate the weak spots of the current reference frame.


The timescales of protoplanetary disk evolution constrained by Gaia membership analysis

by Carlo F. Manara

While protoplanetary disks are known to be the birthplace of planets; the understanding of both how and when planets form and how exactly disks evolve is still missing. One of the main ingredients to decipher how planets form is to know the timescales of their evolution. Previous surveys with Spitzer and other infrared telescopes have shown a general trend of decreasing of the fraction of stars with disks with time; with a typical e-folding time of 3-5 Myr.

However; our recent results based on Gaia DR2 have shown that these infrared-based memberships of young star-forming regions can be highly contaminated by background giants with similar infrared colors as diskless stars (Manara; Prusti et al. 2018). This result; which is also found in several other regions; can change our view of the timescales of disk evolution. I will present the first results we obtained using Gaia DR2 data; and describe how we are performing detailed analyses of the membership of nearby star-forming regions with Gaia astrometric data to refine the disk fraction and thus the disk evolution timescales.


Study of stellar populations using long-period variables in the Gaia era

by Nami Mowlavi, Thomas Lebzelter, Michele Trabucchi, et al.

An all-sky catalog of long-period variables (LPVs) has been published in Gaia data release 2 with 151,761 candidates having G-band variability amplitudes larger than 0.2 magnitudes (Mowlavi et al. 2018). This doubled the number of known such variables at the time of the release. It mainly comprises stars near the tip of the red giant branch and on the asymptotic giant branch (AGB).

In a subsequent Letter published in A&A (Lebzelter et al. 2018); we showed on the sample of Gaia LPVs of the Large Magellanic Cloud how optical Gaia photometry combined with infrared 2MASS photometry allows to identify various classes of AGB stars; including C-rich and O-rich stars; as well as low-mass; intermediate-mass and massive AGB stars. In this contribution; we present the latest developments in our studies of the Gaia Galactic and extra-galactic LPVs; in particular regarding period-luminosity relations and the study of stellar populations. We will conclude on the prospects foreseen for the next Gaia data release.


RR Lyrae stars as standard candles in the Gaia Data Release 2 Era.

by Tatiana Muraveva, Hector Delgado, Gisella Clementini, et al.

On 2018 April 25; the Gaia second data release (DR2); has published positions and multi-band photometry for about 1.7 billion sources; parallaxes and proper motions for about 1.3 billion sources (Gaia Collaboration et al. 2018) alongside a catalogue of more than ~500,000 variable stars of different types that comprises 140,784 RR Lyrae stars (Clementini et al. 2018); for which main characteristic parameters (e.g. period; pulsation mode; mean magnitudes in the Gaia G; GBP and GRP passbands; extinction and photometric metal abundances) were also released.

This provides a huge contribution to the knowledge of the RR Lyrae population in and beyond the Milky Way (MW) and also makes it possible to calibrate their fundamental relations and extend them to the Gaia passbands. We present results of our analysis of about 400 MW RR Lyrae stars for which we use the accurate parallaxes available with Gaia DR2 along with a Bayesian fitting approach to derive new visual absolute magnitude - metallicity (MV - [Fe/H]) and infrared period-luminosity (PL) and PL-metallicity (PLZ) relations, as well as the first luminosity-metallicity relation in the Gaia G-band (MG - [Fe/H]).

The new relations are used to measure absolute magnitudes and distances to RRLs in the Large Magellanic Cloud (LMC); for which trigonometric parallax measurements are hampered by the faint magnitude/large distance. In doing so we also test the Gaia DR2 parallaxes and the effect of zero-point parallax offset on the RRL fundamental relations.


Space distribution; dynamics; and the origin of Galactic black hole X-ray binaries revealed by Gaia DR2

by Anjali Rao, Poshak Gandhi, Michael A.C. Johnson, et al.

Black hole X-ray binaries (BHBs) are key sources in post-stellar evolution and accretion studies. Only about 20 systems are confirmed as hosting black holes so far; yet remain poorly characterized because the majority are optically faint (Gmag>19) and located at heliocentric distances of several kpc. The exquisite precision and sensitivity of Gaia DR2 yields five-parameter astrometric solutions for 11 of these systems. Bayesian distances based upon a custom-tailored prior are found to be in fair agreement with literature estimates derived from a variety of photometric and spectroscopic techniques.

This is encouraging given the faintness of the population; and provides a first geometric test of many literature estimates. Orbital wobble may have been detected in one case (Cygnus X-1). And one case of strong discrepancy is BW Cir; for which previous studies have suggested a distant location at >25 kpc; whereas DR2 instead prefers a geometric distance of <~1 kpc; which would instead make the source one of the nearest of BHBs. Alternatively; at large distances; the space velocity would exceed the Galactic escape speed.

We examine all DR2 flags and other potential biases; but there is no obvious resolution for this source at present. DR2 also allowed measurement of space kinematics of 7 BHBs for the first time; resulting in the identification of BW Cir as a new high velocity Galactic BHB. A weak anti-correlation between kinetic energy of peculiar motion is found against black hole mass; tentatively supporting black hole formation scenarios where natal kicks imparted during supernova core-collapse scale with black hole mass.

Finally; DR2 is used for a dedicated case-study on the origin of the prototypical BHB Cyg X-1. The distance; proper motion; peculiar velocity and relative velocity of Cyg X-1 are found to be consistent with those of the neighboring Cyg OB3 massive star-forming association.


Unlocking galactic Wolf-Rayet stars with Gaia DR2

by Gemma Rate, Paul Crowther

Wolf-Rayet stars are the helium core burning stage of the most massive O stars. They have strong and dense stellar winds; which remove outer hydrogen layers from the atmosphere. This reveals core burning products; mainly nitrogen and carbon; which appear as broad emission lines. The Milky-Way contains a resolved and varied Wolf-Rayet population. As the end stage of very high mass stars; they can reveal much about massive star origins and fates. Their primary formation sites can be studied by determining the fraction in clusters and associations.

The fraction of runaways flung outside the plane also give an insight into how frequently massive stars experience binary interactions during their lives. Additionally; Wolf-Rayet stars are key progenitors of helium free type Ib/c supernovae and trace the environments in which they will likely occur. Well constrained distances to individual stars are required to explore these topics. Past work relied upon distances to Wolf-Rayet members of clusters and associations; which left non-members with unreliable distances.

Precise parallax measurements from Gaia DR2 therefore vastly expand the number of Milky-Way Wolf-Rayet stars with robust distances from just a handful of stars to hundreds. Combined with DR2 proper motions; they offer an improved understanding of clusters; associations and runaways. In this talk; I will present the distances generated from DR2 using a Bayesian method; with a prior tailored for high mass stars. I will also discuss the resulting insights into our closest Wolf-Rayet population. This will include their membership of clusters and associations; the fraction of runaways and their relation to massive star evolution in general.


Invited talk: Gaia and the white dwarf cooling track

by Pier-Emmanuel Tremblay


The vast majority of stars will become white dwarfs at the end of the stellar life cycle. These remnants are precise cosmic clocks owing to their well constrained cooling rates. Gaia Data Release 2 has discovered a quarter million white dwarfs offering unprecedented details on their cooling physics. Most of these objects will be followed-up spectroscopically with WEAVE; SDSS-V; DESI and 4MOST.

By employing their derived stellar parameters, white dwarfs can constrain the stellar formation history in the early developing phases of the Milky Way, the initial mass function in the 1.5 to 8 Solar mass range, and the evolution of planetary systems after the main-sequence.



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.