A 500 pc volume-limited sample of hot subluminous stars. I. Space density, scale height, and population properties

Abstract

We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the Gaia space mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 ($ 77<!PCT!>$) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 ($ are hydrogen-rich sdBs, 65 are sdOBs ($ 21<!PCT!>$), 32 are sdOs ($ 11<!PCT!>$), and 30 are He-sdO/Bs ($ 10<!PCT!>$). Among them, 48 ($ 16<!PCT!>$) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90<!PCT!> complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the Gaia colour-magnitude diagram (CMD). The remaining sources in the sample include cataclysmic variables (CVs), blue horizontal branch stars (BHBs), hot white dwarfs (WDs), and MS stars. We derived the mid-plane density $ $ and scale height $ h z $ for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech$^2$). The best-fit values are $ $ stars/pc3 and $ h z 62$ pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to $ $ stars/pc3. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates.