Hot subdwarf stars from the Hamburg Quasar Survey

Hot subluminous stars (subdwarf B&O; sdB, sdO) are evolved low mass stars originating from red giants that lost their envelope almost entirely. The multitude of observed phenomena imply that several pathways may form hot subdwarfs, most involving close binary channels. The Hamburg Quasar Survey (HQS) has led to the discovery of many faint blue stars, including hot subdwarf stars. Many of the HQS-sdB stars have been studied in detail, but analyses of the helium-rich sdOB and sdO stars are lacking. The recent development of model spectra calulated from model atmopheres in local thermodynamic equilibrium (LTE) allowing for non-LTE departures (hybrid LTE/NLTE model spectra, the 2nd generation Bamberg model grids) enables us to improve the spectroscopic analyses of sdB stars as well as of the previously unstudied sdO stars allowing precise atmospheric parameters to be derived, while consistently accounting for parameter correlations and systematic uncertainties. The Gaia mission provided astrometric data of unprecedented quality, which allow fundamental stellar parameters to be derived from atmospheric parameters via parallax measurements. We used spectral energy distributions to identify composite-colour sdB binaries and present the result of detailed spectroscopic analyses of 122 non-composite subdwarfs from the HQS to identify potential evolutionary pathways. Comparison to evolutionary tracks both in the Kiel (Teff-log g) and the physical Hertzsprung–Russell (Teff–logL) diagram finds the location of the sdB stars on the extreme horizontal branch (EHB). Their derived mass distribution and median mass of 0.45 M⊙ are consistent with the canonical EHB mass. We revisited the sample of known pulsating HQS-sdB stars and find no significant differences between their mass distributions and those of sdB stars that do not pulsate. The helium-rich sdOB and sdO stars are found near the helium main sequence (He-MS). The derived mass distribution of the extremely He-rich subdwarfs is broader (0.48–1.05 M⊙) and peaks at a median of 0.70 M⊙, significantly larger than those of the hydrogen-rich stars. Intermediate He-rich subdwarfs are also He-MS stars, but of lower mass (0.55 M⊙) than the extremely He-rich subdwarfs. This strongly supports the merger scenario for the origin of He-rich sdO stars, in which two helium white dwarfs merge following orbital decay driven by gravitational-wave emission, producing a He-rich sdO or sdOB star. Comparing results from similar studies, we speculate that older populations produce more massive helium white dwarfs mergers.