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Preprint 10c/12

On the Evolution of O(He)-Type Stars

N. Reindl (1), T. Rauch (1), K. Werner (1), and J. W. Kruk (2)

(1) Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tuebingen, Germany
(2) NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

To be published in: UV Astronomy, HST and Beyond, Kaua'i, Hawai'i, USA, June 18-21, 2012

Abstract. O(He) stars represent a small group of four very hot post-AGB stars whose atmospheres are composed of almost pure helium. Their evolution deviates from the hydrogen-deficient post-AGB evolutionary sequence of carbon-dominated stars like e.g. PG 1159 or Wolf- Rayet stars. While (very) late thermal pulse evolutionary models can explain the observed He/C/O abundances in these objects, they do not reproduce He-dominated surface abundances. Currently it seems most likely that the O(He) stars originate from a double helium white dwarf merger and so they could be the successors of the luminous helium-rich sdO-stars. An other possibility is that O(He)-stars could be successors of RCB or EHe stars. We present the results of a non-LTE spectral analysis of all four O(He) stars, based on optical, FUSE, and HST/COS observations.

Preprint (1,55 Mb PDF file including figures)

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