Hot subdwarf stars from the Hamburg Quasar Survey

Atmospheric and fundamental parameters for 122 non-composite hot subdwarfs from the Hamburg Quasar Survey. The extremely helium-rich sdOs come out markedly more massive than their hydrogen-rich counterparts, which points at a white dwarf merger origin.

Abstract

Hot subluminous stars of spectral types sdB and sdO are evolved low-mass stars that lost almost their entire envelope while on the red giant branch. The Hamburg Quasar Survey turned up a large number of them as a by-product of its search for quasars, and this sample has until now been analysed only in part.

We derive atmospheric and fundamental parameters for 122 non-composite subdwarfs, using hybrid LTE/NLTE model spectra together with Gaia astrometry to put the stars on an absolute scale.

The hydrogen-rich sdB stars sit on the extreme horizontal branch with a median mass of 0.45 M_☉, in line with canonical expectations. The helium-rich sdOB and sdO stars cluster near the helium main sequence, and the extremely helium-rich subdwarfs among them are substantially more massive, with a median of 0.70 M_☉ and a range of 0.48 to 1.05 M_☉. Those masses support a merger scenario for the formation of He-rich sdO stars, in which two helium white dwarfs merge after gravitational-wave-driven orbital decay; older stellar populations would then be expected to yield the more massive mergers.