A 500 pc volume-limited sample of hot subluminous stars

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.