Broadening of Ly α by neutral helium in DBA white dwarfs

Traces of photospheric hydrogen are detected in at least half of all white dwarfs with heliumdominated atmospheres through the presence of Hα in high-quality optical spectroscopy. Previous studies have noted significant discrepancies between the hydrogen abundances derived from Hα and Ly α for a number of stars where ultraviolet spectroscopy is also available. We demonstrate that this discrepancy is caused by inadequate treatment of the broadening of Ly α by neutral helium. When fitting Hubble Space Telescope far-ultraviolet spectroscopy of 17 DB white dwarfs using our new line profile calculations, we find good agreement between log (NH/NHe) measured from Ly α and Hα. Larger values of log (NH/NHe) based on Ly α are still found for three stars, which are among the most distant in our sample, and we show that a small amount of interstellar absorption from neutral hydrogen can account for this discrepancy