A 500 pc volume-limited sample of hot subluminous stars

Year: 2026

Type: article

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 (Teff, 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.

Source: Springer Link (Chiba Institute of Technology)

DOI: 10.1051/0004-6361/202558123/pdf

Institutions: University of Potsdam, University of Warwick, Friedrich-Alexander-Universität Erlangen-Nürnberg, University of Valparaíso, KU Leuven, Thüringer Landessternwarte Tautenburg, Heidelberg University, Universitat Politècnica de Catalunya, European Southern Observatory, Radboud University Nijmegen, Max Planck Institute for Astrophysics, Aarhus University, Nordic Optical Telescope, UNSW Sydney, Australian Defence Force Academy, University of Göttingen

Language: English

Topic: Stellar, planetary, and galactic studies

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: green

Funders: National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, European Space Agency, University of Warwick, European Commission, Deutsche Forschungsgemeinschaft, Grantová Agentura České Republiky, Fonds Wetenschappelijk Onderzoek, Nuclear Safety and Security Commission, KU Leuven, China National Textile and Apparel Council, Vlaamse regering, Instituto de Astrofísica de Andalucía, Agencia Estatal de Investigación

OpenAlex ID: W7124447495