A 500 pc volume-limited sample of hot subluminous stars

I. Space density, scale height, and population properties^★

¹ Institute for Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
e-mail: hdawson@astro.physik.uni-potsdam.de
² Dr. Remeis-Sternwarte and ECAP, Astronomical Institute, University of Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ Department of Physics, University of Warwick, Gibet Hill Road, Coventry CV4 7AL, UK
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁵ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁶ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
⁷ European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile
⁸ Astronomical Institute of the Czech Academy of Sciences, 251 65 Ondřejov, Czech Republic
⁹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁰ Max Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany
¹¹ Recogito AS, Storgaten 72, 8200 Fauske, Norway
¹² Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
¹³ Universitat Politècnica de Catalunya, Departament de Física, c/ Esteve Terrades 5, 08860 Castelldefels, Spain
¹⁴ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁶ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
¹⁷ Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-900 Porto-Alegre, RS, Brazil
¹⁸ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹⁹ Isaac Newton Group of Telescopes, Apartado de Correos 368, 38700 Santa Cruz de La Palma, Spain
²⁰ Department of Physics and Astronomy, Aarhus University, Munkegade 120, 8000 Aarhus C, Denmark
²¹ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
²² Nicolaus Copernicus Astronomical Centre, ul. Rabiańska 8, 87‐100 Toruń, Poland
²³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
²⁴ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 19 October 2023
Accepted: 21 February 2024

Abstract

We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the Gaia space mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 (~77%) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 (~58%) are hydrogen-rich sdBs, 65 are sdOBs (~21%), 32 are sdOs (~11%), and 30 are He-sdO/Bs (~10%). Among them, 48 (~16%) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the Gaia colour-magnitude diagram. The remaining sources in the sample include cataclysmic variables, blue horizontal branch stars, hot white dwarfs, and MS stars. We derived the mid-plane density ρ₀ and scale height h_z for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech²). The best-fit values are ρ₀ = 5.17 ± 0.33 × 10⁻⁷ stars pc⁻³ and h_z = 281 ± 62 pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to ρ₀ = 6.15_−0.53^+1.16 × 10⁻⁷ stars pc⁻³. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates.