A 500 pc volume-limited sample of hot subluminous stars III. The short-period binary population

Orbits for 34 newly found close binaries in the 500 pc sample, giving a short-period binary fraction of 34.7% across 301 hot subdwarfs and a volume-complete estimate of their contribution to the Galactic Type Ia rate.

Abstract

Hot subluminous stars are mainly formed through binary interaction, with the short-period population carrying the clearest record of common-envelope evolution. We present the results of a radial velocity survey of the 500 pc volume-limited sample of hot subluminous stars defined from Gaia DR3, covering 301 objects.

We identify 50 new radial-velocity variable systems and derive orbital solutions for 34 of them, with periods spanning 0.07 to 22 days. Combining these with the orbits already in the literature and correcting for detection incompleteness, we find an overall short-period binary fraction of 34.7^+2.8_−2.9% for the full sample. Reflection-effect and ellipsoidal-modulation systems are characterised alongside the spectroscopic orbits.

The binary fractions separate by spectral class: sdOB stars show roughly half the short-period binary fraction of the sdB and sdO populations. Underluminous subdwarfs are found exclusively with white dwarf companions, which are on average more massive and at shorter orbital periods than the companions of the rest of the sample. From volume-complete birthrates we estimate that these binaries could contribute up to 2.5^+0.7_−0.5% of the observed Galactic Type Ia supernova rate.