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%) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 (~58%) are hydrogen-rich sdBs, 65 are sdOBs (~21%), 32 are sdOs (~11%), and 30 are He-sdO/Bs (~10%). Among them, 48 (~16%) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the Gaia colour-magnitude diagram. The remaining sources in the sample include cataclysmic variables, blue horizontal branch stars, hot white dwarfs, and MS stars. We derived the mid-plane density ρ<SUB>0</SUB> and scale height h<SUB>z</SUB> for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech<SUP>2</SUP>). The best-fit values are ρ<SUB>0</SUB> = 5.17 ± 0.33 × 10<SUP>−7</SUP> stars pc<SUP>−3</SUP> and h<SUB>z</SUB> = 281 ± 62 pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to ρ<SUB>0</SUB> = 6.15<SUB>−0.53</SUB><SUP>+1.16</SUP> × 10<SUP>−7</SUP> stars pc<SUP>−3</SUP>. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates. <P />Tables A.1-A.3 are available at the CDS ftp to <A href=https://cdsarc.cds.unistra.fr>cdsarc.cds.unistra.fr</A> (ftp://130.79.128.5) or via <A href=https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/686/A25>https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/686/A25</A>