Abstract
Spectroscopic analyses of blue horizontal branch stars, subluminous B- and O-stars are reviewed. These classes of stars trace stellar evolution from the horizontal branch towards the white dwarf cooling sequence. The resulting atmospheric parameters and abundances are used to obtain constraints on the evolutionary status of the different classes of stars. The sdB stars form a homogeneous group and can be identified with models of the extended horizontal branch. Abundance anomalies (deficiency of helium and some metals, enrichment of 3He) observed in Horizontal Branch stars and sdB stars are caused by atmospheric diffusion. The class of subluminous O stars is much less homogenous and two subclasses can be defined: the “compact” sdO stars probably evolved from the extended horizontal branch and are hence successors of the sdBs, whereas some sdOs of relatively low gravity are in a post-AGB stage of evolution. Hot subdwarfs in binary systems can be formed by case B or case C mass transfer.
Published Version
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