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Paper: Subluminous O Stars - Origin and Evolutionary Links
Volume: 391, Hydrogen-Deficient Stars
Page: 245
Authors: Heber, U.
Abstract: Hot subluminous stars can be roughly divided into B- and Otypes. 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.
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