The pattern of heavy element abundances in hot DA white dwarfs

We have carried out a systematic series of abundance measurements of hot DA white dwarfs in the temperature range 20,000-75,000K, combining all the available spectra from both our own observation programmes and the far-UV archives with new model calculations (Barstow 2001 283). In particular, we have addressed the abundance patterns for the hottest stars for the first time, showing that they are similar to objects like G191-B2B. The abundances observed in the cooler white dwarfs (T<50,000 K) are something of a mystery. Some of the patterns can be explained by self-consistent levitation-diffusion calculations, where the radiation pressure supports certain elements against the downward pull of gravity. However, there is then a problem in understanding the appearance of apparently pure H atmospheres and the presence of heavy elements in cool DAs, where the radiative levitation mechanism is no longer effective. In the latter case, the stars may be undergoing active, low-level, accretion. Note that there is strong evidence that the 35,000K DA star GD394, which has the highest Si abundance known in any white dwarf, has a companion. Hence, the presence of heavy elements in the cooler DAs may be a signature of binarity, where the companion is too cool/low mass to detect in visible band spectra.