A 500 pc volume-limited sample of hot subluminous stars II. Atmospheric parameters, mass distribution, and kinematics

Abstract

We present a quantitative spectroscopic and kinematic analysis of the volume-complete sample of hot subluminous stars within 500 pc of the Sun, assembled from accurate parallaxes in Gaia DR3. In total, 3226 spectra of 253 hot subdwarfs were analysed to derive effective temperatures, surface gravities, helium abundances and radial velocities, while spectral energy distributions combined with Gaia parallaxes gave stellar radii, luminosities and masses.

The atmospheric parameters place sdB and sdO stars in consistent alignment in the Kiel diagram when compared to theoretical evolutionary models. About 10% of the sample sits below the 0.45 M_☉ zero-age extreme horizontal branch in both the Kiel and Hertzsprung-Russell diagrams, and these objects likely descend from intermediate-mass progenitors of 1.8 to 8 M_☉. The overall mass distribution peaks at 0.48 ± 0.12 M_☉, while the below-EHB stars peak at 0.43 ± 0.10 M_☉, consistent with the non- or semi-degenerate helium ignition expected of intermediate-mass stars. Interpolating EHB and post-EHB tracks yields mass distributions that agree with the ones from the SEDs and parallaxes. Assuming masses between 0.40 and 0.50 M_☉, the post-EHB birthrate comes out 2 to 3 times higher than the EHB birthrate, suggesting overestimated EHB lifetimes or contamination from additional formation channels.

Kinematically, 86 ± 2% of the stars belong to the Galactic thin disk, with 13 ± 1% in the thick disk and 1 ± 1% in the halo. The below-EHB population is found exclusively in the thin disk, the only Galactic component young enough to host intermediate-mass progenitors. Its absence from other large samples suggests that non-degenerate formation channels play a more prominent role in the Galactic disk than previously assumed.