Spectroscopic Analyses of Hot Extreme Helium Stars

Abstract

ABSTRACT. Spectroscopic fine analyses of hot extreme helium stars are reviewed. The chemical composition is discussed in detail and conclusions as to the nuclear history of the atmospheric material are drawn. Evidence of inhomogeneities among the hot extreme helium stars is presented and it is concluded that the extreme helium stars can be found in the halo population (BD+10°2179) as well as in the disc population. Their mass loss rates are of the same order of magnitude (10 M /yr) as those of normal stars. Four helium rich sdO stars are identified'as possible descendants of the B-type extreme helium stars.