2026A&A...707A...6D

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^+0.14M_⊙, while hot subdwarfs below the EHB peak at 0.43_–0.09^+0.11M_⊙, 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.