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EXTREME HELIUM STARS

The late stages of stellar evolution are characterized by extremes. Stellar structure and surface properties change much more rapidly than during earlier phases of evolution. A star will reach its highest luminosity and, often, its highest effective temperature shortly before it finally becomes a white dwarf. Mass-loss and mixing may expose highly-processed material on the stellar surface, resulting in the formation of chemically peculiar stars. The latter are distributed over most of the Hertzsprung-Russell diagram; one of the most extreme examples is provided by the extreme helium stars (EHes) - early-type supergiants practically void of hydrogen in their atmospheres.

The remarkable hydrogen-deficiency of an EHe is demonstrated by the weakness of its H$\gamma$ line. In the case of a normal B star, such as $\gamma$ Peg, this typically broad line dominates the local spectrum. In the case of the extreme helium star LSE78, H$\gamma$ is almost completely replaced by a blend of Siii and Oii absorption lines. The effective temperatures of these two stars are 21,500K and 18,000K respectively (Peters 1976, Jeffery 1993), so that the ionization balance of their photospheres will not be dissimilar. Hydrogen constitutes less than $10^{-5}$ parts by number in LSE78. A direct consequence of the low hydrogen abundance is that the continuum opacity, normally dominated by hydrogen, is reduced. Although the abundances of species other than helium and carbon are not significantly different from solar, the metal line spectrum is correspondingly magnified several fold.

This paper introduces the principal properties of extreme helium stars. Since their surface compositions imply an unusual evolutionary history, I review the principal scenarios currently considered. Stellar mass is critical to a correct interpretation of stellar evolution, so methods for its measurement are compared. The importance of pulsations in providing masses and other characteristics of helium star evolution is demonstrated, with particular reference to the radial pulsations in V652Her and BXCir.