The EUV spectrum of the unique bare stellar core H 1504 + 65 ⋆

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 Teff=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 3α processed matter and corroborates our understanding of H1504+65 as a C-O stellar core which has lost its entire Hand He-rich envelopes.