Broadening of Ly α by neutral helium in DBA white dwarfs

Abstract

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.