Atmospheres and Abundances of Blue Horizontal Branch Stars and Related Objects

Abstract

Spectroscopic analyses of blue horizontal branch stars, subluminous B- and O-stars and PG 1159 stars are reviewed. These classes of stars trace stellar evolution from the horizontal branch towards the white dwarf cooling sequence. The impact of Non-LTE model atmospheres for the analyses of sdO and PG 1159 stars is outlined. 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 do not give hints to the stars' evolutionary history but 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. The third class, the PG 1159 stars, are amongst the hottest stars known with effective temperatures exceeding 100,000 K and are immediate progenitors of the white dwarfs. The abundance pattern is dominated by carbon and helium with a significant admixture of oxygen, whereas hydrogen and nitrogen are trace elements only. As in the case of the (helium rich) sdO stars these abundances indicate that nuclear processed material is exposed at the stellar surface. The origin of the PG 1159 stars as well as of the “low gravity” sdO stars can be explained by the born-again post-AGB star scenario of Iben et al. (1983).