The GALAH survey: multiple stars and our Galaxy

I. A comprehensive method for deriving properties of FGK binary stars^★

¹ Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
e-mail: gregor.traven@astro.lu.se
² Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Brussels, Belgium
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, I50125 Firenze, Italy
⁴ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
⁵ Institute for Advanced Study, Princeton, NJ 08540, USA
⁶ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁷ Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁸ ARC Centre of Excellence for All Sky Astrophysics in Three Dimensions (ASTRO-3D), Canberra, ACT 2611, Australia
⁹ Research School of Astronomy & Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁰ Sydney Institute for Astronomy, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia
¹¹ Department of Physics & Astronomy, Macquarie University, Sydney, NSW 2109, Australia
¹² School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
¹³ Research Centre in Astronomy, Astrophysics & Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia
¹⁴ Max Planck Institute for Astronomy (MPIA), Koenigstuhl 17, 69117 Heidelberg, Germany
¹⁵ Monash Centre for Astrophysics (MoCA) and School of Physics and Astronomy, Monash University, Clayton Vic 3800, Australia
¹⁶ Faculty of Information Technology, Monash University, Clayton 3800, Victoria, Australia
¹⁷ INAF, Vicolo dell’Osservatorio, 5, 35122 Padova, PD, Italy
¹⁸ Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
¹⁹ INAF Astronomical Observatory of Padova, 36012 Asiago (VI), Italy
²⁰ University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350, Australia

Received: 12 January 2020
Accepted: 12 April 2020

Abstract

Context. Binary stellar systems form a large fraction of the Galaxy’s stars. They are useful as laboratories for studying the physical processes taking place within stars, and must be correctly taken into account when observations of stars are used to study the structure and evolution of the Galaxy. The advent of large-scale spectroscopic and photometric surveys allows us to obtain large samples of binaries that permit characterising their populations.

Aims. We aim to obtain a large sample of double-lined spectroscopic binaries (SB2s) by analysis of spectra from the GALAH survey in combination with photometric and astrometric data. A combined analysis will provide stellar parameters of thousands of binary stars that can be combined to form statistical observables of a given population. We aim to produce a catalogue of well-characterised systems, which can in turn be compared to models of populations of binary stars, or to follow-up individual systems of interest.

Methods. We obtained a list of candidate SB2 systems from a t-distributed stochastic neighbour embedding (t-SNE) classification and a cross-correlation analysis of GALAH spectra. To compute parameters of the primary and secondary star, we used a Bayesian approach that includes a parallax prior from Gaia DR2, spectra from GALAH, and apparent magnitudes from APASS, Gaia DR2, 2MASS, and WISE. We used a Markov chain Monte Carlo approach to sample the posterior distributions of the following model parameters for the two stars: T_eff[1,2], logg_[1,2], [Fe/H], V_r[1,2], v_mic[1,2], v_broad[1,2], R_[1,2], and E(B−V).

Results. We present results for 12 760 binary stars detected as SB2s. We construct the statistical observables T₁∕T₂, ΔV_r, and R₁∕R₂, which demonstrate that our sample mostly consists of dwarfs, with a significant fraction of evolved stars and several dozen members of the giant branch. The majority of these binary stars is concentrated at the lower boundary of the ΔV_r distribution, and the R₁∕R₂ ratio is mostly close to unity. The derived metallicity of our binary stars is statistically lower than that of single dwarf stars from the same magnitude-limited sample.

Conclusions. Our sample of binary stars represents a large population of well-characterised double-lined spectroscopic binaries that are appropriate for statistical studies of the binary populations. The derived stellar properties and their distributions show trends that are expected for a population of close binary stars (a < 10 AU) detected through double lines in their spectra. Our detection technique allows us to probe binary systems with mass ratios 0.5 ≤q ≤ 1.