Subluminous O Stars - Origin and Evolutionary Links.pdf

a r X i v : 0 8 0 4 .1 5 2 7 v 1 [ a s t r o - p h ] 9 A p r 2 0 0 8 Hydrogen-Deficient Stars ASP Conference Series, Vol. xxx, 2008 Werner Rauch Subluminous O Stars – Origin and Evolutionary Links Uli Heber Dr. Remeis-Sternwarte, Universita?t Erlangen-Nu?rnberg, Sternwartstr. 7, 96049 Bamberg, Germany Abstract. Hot subluminous stars can be roughly divided into B- and O- types. Unlike the latter many sdBs are found in close binaries, indicating that binary evolution plays a vital role. Recent NLTE spectral analyses revealed that an evolutionary link between sdB stars and sdO stars is plausible only for the helium-deficient sdO stars, i.e. they are the likely successors to sdB stars. The atmospheric properties of helium-enriched sdO stars can be explained by the late hot flasher as well as by the white-dwarf merger scenarios, although both models do not match the observed properties of helium-enriched sdO stars in detail. The white dwarf merger scenario is favoured because it naturally explains the scarcity of close binaries amongst helium-enriched sdO stars. A hyper-velocity sdO star moving so fast that it is unbound to the Galaxy has probably been ejected by the super-massive black hole in the Galactic centre. 1. Hot Subluminous Stars Hot subluminous stars are an important population of faint blue stars at high Galactic latitudes closely related to the horizontal branch. A proper spectral classification of hot subluminous stars is rendered difficult by the diversity of the helium line spectra. They can be grouped roughly into the cooler sdB stars, whose spectra typically display no or only weak helium lines, and the hotter sdO stars, which have a higher helium abundance on average and can even be dominated by helium. The former have recently been studied extensively because they are common enough to account for the UV excess observed in early-type galaxies. Pulsating sdB stars are important tools for asteroseismology (Charpinet et al. 2004), and sdB stars i