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

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--150A) is dominated by strong OVI lines and we can identify a number of NeVII lines. EUVE and ROSAT data can be fitted with models between Teff=170000 K and 200000 K. We derive an extraordinarily high neon abundance (2%--5% by mass, i.e. 20--50 times solar) which we confirm by an optical NeVII 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.