Spectral analysis of O(He)-type post-AGB stars

Abstract

We present an analysis of two recently discovered, very hot hydrogen-deficient post-AGB stars of spectral type O(He), namely HS 1522+6615 and HS 2209+8229, by means of state-of-the-art NLTE model atmospheres based on new optical, UV, and X-ray observations. In the spectra of HS 1522+6615 we discovered a variability of the \Ionw{O}{6}{5290} line complex on a time scale of few days. We also report on the discovery of a ring structure in the planetary nebula K 1-27 which has an O(He)-type central star. We discuss how the group of O(He) stars fits in our picture of hydrogen-deficient post-AGB stellar evolution. Based on observations obtained at the German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie Heidelberg jointly with the Spanish National Commission for Astronomy; collected at the European Southern Observatory, La Silla, Chile; on observations made with ROSAT, retrieved from the archive; and on observations made with the International Ultraviolet Explorer (IUE) and retrieved from the IUE Final Archive; this research has made use of the SIMBAD database, operated at CDS, Strasbourg, France