Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany
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Preprint 05/99

The EUV spectrum of the unique bare stellar core H1504+65

Klaus Werner (1), Burkhard Wolff (2)

(1) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2) Institut für Theoretische Physik und Astrophysik, Universität Kiel, D-24098 Kiel, Germany

To appear in: Astronomy and Astrophysics

Abstract. We performed a spectral analysis of the EUVE spectrum and ROSAT data of the unique object H1504+65, which is an extremely hot post-AGB star entering the white dwarf cooling sequence. It is the only pre-white dwarf known, whose surface is free of hydrogen and helium, hence, it represents the bare core of a former AGB star. The EUV spectrum (75-150Å) is dominated by strong O VI lines and we can identify a number of Ne VII lines.
EUVE and ROSAT data can be fitted with models between T_eff=170.000 K and 200.000 K. We derive an extraordinarily high neon abundance (2%-5% by mass, i.e. 20-50 times solar) which we confirm by an optical Ne VII line detected in a Keck HIRES echelle spectrum. This abundance is expected for 3-alpha processed matter and corroborates our understanding of H1504+65 as a C-O stellar core which has lost its entire H- and He-rich envelopes.

Key words: Stars: abundances - Stars: atmospheres - Stars: evolution - Stars: AGB and post-AGB - white dwarfs - Stars: individual: H1504+65

Paper (92k gzip'ed Postscript including figures)

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