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Astron. Astrophys. 338, 651-660 (1998)


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

T. Rauch ** 1, S. Dreizler[FORMULA] 1 and B. Wolff 2

1 Institut für Astronomie und Astrophysik der Universität, D-72076 Tübingen, Germany
2 Institut für Theoretische Physik und Astrophysik der Universität, D-24098 Kiel, Germany

Received 25 May 1998 / Accepted 17 July 1998

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 [FORMULA] 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.

Key words: stars: abundances – stars: evolution – stars: individual: HS 1522+6615 – stars: individual: HS 2209+8229 – stars: AGB and post-AGB – planetary nebulae: individual: K 1-27

* 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
** Visiting astronomer, Calar Alto, Spain

Send offprint requests to: T. Rauch, (rauch@astro.uni-tuebingen.de)

© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998

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