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We have carried out a non-LTE analysis of X-ray and ultraviolet observations of the hot pre-white dwarf H1504+65. The X-ray observations were carried out with the ROSAT Position Sensitive Proportional Counter (PSPC) and the Wide Field Camera (WFC) while the UV observations were obtained using the Faint Object Spectrograph (FOS) onboard the Hubble Space Telescope. The ROSAT observations revealed H1504 to be an extremely bright soft X-ray source, its spectrum peaking between 0.10-.015 keV. The UV spectrum of H1504 is characterized by broad absorption lines of C IV and O VI superimposed onto a Rayleigh-Jeans continuum.
The non-LTE models we employed in this study were designed to test for the presence of He in the photosphere of H1504+65, and are similar to the models used by Werner (1991) in a previous study of optical data of H1504+65. Although problems with the PSPC detector response did not enable us to determine accurately the composition of H1504+65, we were able to rule out the presence of He. Thus our results support the previous conclusions of Werner (1991) that H1504+65 is entirely devoid of He, its atmosphere instead dominated by C and O. Despite the lack of He, H1504+65 is still best interpreted as being an extreme member of the PG1159 class of object. That is, H1504+65 represents an evolutionary link between the O VI central stars of planetary nebulae and the non-DA white dwarfs.
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