A 500 pc volume-limited sample of hot subluminous stars

II. Atmospheric parameters, mass distribution, and kinematics

¹ Institute for Physics and Astronomy, University of Potsdam Karl-Liebknecht-Str. 24/25 14476 Potsdam, Germany
² Department of Physics, University of Warwick Gibet Hill Road Coventry CV4 7AL, UK
³ Dr. Remeis-Sternwarte and ECAP, Astronomical Institute, University of Erlangen-Nürnberg Sternwartstr. 7 D-96049 Bamberg, Germany
⁴ Instituto de Física y Astronomía, Universidad de Valparaíso Gran Bretaña 1111 Playa Ancha Valparaíso 2360102, Chile
⁵ European Southern Observatory Alonso de Cordova 3107 Santiago, Chile
⁶ Institute of Astronomy, KU Leuven Celestijnenlaan 200D B-3001 Leuven, Belgium
⁷ Thüringer Landessternwarte Tautenburg Sternwarte 5 D-07778 Tautenburg, Germany
⁸ Landessternwarte Heidelberg, Zentrum fur Astronomie, Ruprecht-Karls-Universitat Konigstuhl 12 69117 Heidelberg, Germany
⁹ Universitat Politècnica de Catalunya, Departament de Física c/ Esteve Terrades 5 08860 Castelldefels, Spain
¹⁰ Department of Astrophysics/IMAPP, Radboud University P O Box 9010 NL-6500 GL Nijmegen, The Netherlands
¹¹ Max Planck Institut für Astrophysik Karl-Schwarzschild-Straße 1 85748 Garching bei München, Germany
¹² Recogito AS Storgaten 72 N-8200 Fauske, Norway
¹³ Nordic Optical Telescope Rambla José Ana Fernández Pérez 7 ES-38711 Breña Baja, Spain
¹⁴ Department of Physics and Astronomy, Aarhus University Munkegade 120 DK-8000 Aarhus C, Denmark
¹⁵ Isaac Newton Group of Telescopes Apartado de Correos 368 E-38700 Santa Cruz de La Palma, Spain
¹⁶ Department of Physics and Astronomy, University of Sheffield Sheffield S3 7RH, UK
¹⁷ Nicolaus Copernicus Astronomical Centre ul. Rabiańska 8 87-100 Toruń, Poland
¹⁸ School of Science, University of New South Wales, Australian Defence Force Academy Canberra ACT 2600, Australia
¹⁹ Leuven Gravity Institute, KU Leuven Celestijnenlaan 200D box 2415 3001 Leuven, Belgium
²⁰ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen Friedrich-Hund-Platz 1 37077 Göttingen, Germany

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Received: 14 November 2025
Accepted: 4 January 2026

Abstract

We present a quantitative spectroscopic and kinematic analysis of a volume-complete sample of hot subluminous stars within 500 pc of the Sun, assembled using accurate parallax measurements from the Gaia space mission Data Release 3 (DR3). In total, 3226 spectra of 253 hot subdwarf stars were analysed to derive atmospheric parameters (T_eff, log g, and helium abundance) and radial velocities. Spectral energy distributions (SEDs) with Gaia parallaxes were used to measure stellar radii, luminosities, and masses. The derived atmospheric parameters reveal a consistent alignment between sdB and sdO stars in the Kiel diagram when compared to theoretical evolutionary models. Notably, we find a substantial population (about 10%) of hot subdwarfs located below the 0.45 M_⊙ zero-age extreme horizontal branch (EHB) in both the Kiel and Hertzsprung-Russell diagrams (HRDs), which likely originate from intermediate-mass progenitors (1.8–8 M_⊙). The overall mass distribution peaks at 0.48_−0.10^+0.14 M_⊙, while hot subdwarfs below the EHB peak at 0.43_−0.09^+0.11 M_⊙, supporting a scenario of non- or semi-degenerate helium ignition for these objects characteristic of intermediate-mass stars. Interpolation of EHB and post-EHB tracks yields theoretical mass distributions consistent with our estimates based on SED and parallax. By assuming a mass range between 0.40 and 0.50 M_⊙ during the interpolation, we further find that the post-EHB birthrate in our sample is 2–3 times higher than the EHB birthrate, which may suggest overestimated EHB lifetimes in theoretical tracks or contamination from other formation and evolutionary channels. Our kinematic analysis shows that 86 ± 2% of the stars belong to the Galactic thin disk, with 13 ± 1% and 1 ± 1% associated with the thick disk and halo, respectively. The below-EHB population is exclusively found in the thin disk, the only Galactic population young enough to harbour intermediate-mass stars. The below-EHB population seems to be absent in other large samples, which generally include more thick disk and halo members. These findings suggest that non-degenerate formation channels may play a more prominent role in the Galactic disk than previously thought.