Composition and evolution of hot white dwarfs and their progenitors

A study of the hottest known white dwarfs and their immediate progenitors, the old central stars of planetary nebulae (CSPN), has been carried out with HST/STIS, the Far-UV Spectroscopic Explorer (FUSE) and ground-based telescopes. To complement these observations we have developed new stellar atmosphere models accommodating higher temperatures and including higher ionization stages of important heavy elements. Using these we can now make self-consistent measurements of T and log g , taking into account the composition of the white dwarf atmosphere. For the first time, we have adopted a completely objective approach by using a spectral fitting procedure, involving a chi-squared minimisation technique, which allows us to determine formal errors for measured abundances and, consequently, make statistically valid comparisons of the measurements of individual stars in the sample. As part of this project, we have also obtained the archival data of many cooler stars and reanalysed these with the new models for internal consistency. So far we have studied 20 stars in detail, but this project is a continuing one, with much data (particularly from FUSE cycle 2) to obtain and/or analyse.