Hubble Space Telescope spectroscopy of hot helium–rich white dwarfs: metal abundances along the cooling sequence

Metal abundances are the indicators of the chemical evolution in white dwarfs, which is dominated by the element separation due to the strong gravitational field. A reliable analysis and interpretation requires high resolution and high signalto-noise UV spectroscopy. For hot helium rich DO white dwarfs this is currently only feasible with the Hubble Space Telescope. In this paper I report on our HST spectroscopy of DO white dwarfs and describe our model atmospheres employed for the analysis. This includes an introduction to our new selfconsistent, chemically stratified non-LTE model atmospheres, which take into account gravitational sedimentation and radiative levitation. The results of the analysis shows that DO white dwarfs can best be fitted with chemically homogeneous models, whereas stratified models show significant deviations. Several possible reasons for this unexpected result are discussed. At the current stage, weak mass loss is the most plausible explanation.