UV and soft X-ray observations of central stars of planetary nebulae and hot white dwarfs.

The first IUE SWP high-resolution spectral atlas of central stars of planetary nebulae and hot white dwarfs is presented. Its primary aim is as a set of comparison spectra, with which it is shown that LS V +46 21, the postulated central star of S 216, has a temperature Teff = 80, 000K + 10,000K, and log g ~ 7. These are the characteristics that had been predicted from studies of the nebula itself. The claim of Feibelmann and Bruhweiler (1990) that only those central stars with Fe VII absorption lines show N I has been refuted. Many of the spectra listed by these authors as lacking both ions have continua that are either non-existent or severely under-exposed. Of the remainder, all show N I, and the presence of Fe VII correlates with photospheric temperature. Subsequently, it was found that there is a correlation between the equivalent widths of the N I and Si II 1260A lines and the surface gravity - and thus the age - of the star. This turned out to be a selection effect, with the younger stars both more luminous and sparser, and thus seen behind a larger interstellar column. EUV/soft X-ray data from five central stars and hot white dwarfs has been analysed. For the three with the largest number of observations, opacity sources used in current model atmospheres programs are insufficient. With both H1504+65 and NGC 246, the last two ionisation stages of C and O need to be included, as does Fe V for the hot DA white dwarf G191-B2B. Over twenty lines of this species were detected when five IUE SWP high-resolution spectra of G191-B2B were co-added. The excitation potential of these lines requires a photospheric origin. Consequently, if an observed blueshift of the doublets of N V and C IV with respect to Fe V by ~ 5km s-1 is real, it would suggest that both ions may be formed in a slow wind rather than in the photosphere.