Evidence for an external origin of heavy elements in hot DA white dwarfs

We present a series of systematic abundance measurements for 89 hydrogen atmosphere (DA-type) white dwarfs with temperatures spanning 16 000-77 000 K drawn from the FUSE spectral archive. This is the largest study to date of white dwarfs where radiative forces are significant, exceeding our earlier work, based mainly on International Ultraviolet Explorer and Hubble Space Telescope data, by a factor 3. Using heavy element blanketed non-local thermodynamic equilibrium stellar atmosphere calculations, we have addressed the heavy element abundance patterns making completely objective measurements of abundance values and their error ranges using a chi(2) fitting technique. We are able to establish the broad range of abundances seen in a given temperature range and establish the incidence of stars which appear, in the optical, to be atmospherically devoid of any material other than H. We compare the observed abundances to predictions of radiative levitation calculations, revealing little agreement. We propose that the supply of heavy elements is accreted from external sources rather than being intrinsic to the star. These elements are then retained in the white dwarf atmospheres by radiative levitation, a model that can explain both the diversity of measured abundances for stars of similar temperature and gravity, including cases with apparently pure H envelopes, and the presence of photospheric metals at temperatures where radiative levitation is no longer effective.