Broadening of Ly alpha by neutral helium in DBA white dwarfs

TL;DR

This study resolves previous discrepancies in hydrogen abundance measurements in helium-dominated white dwarfs by improving the modeling of Ly alpha line broadening due to neutral helium, leading to consistent results across different spectral lines.

Contribution

The paper introduces a new line profile calculation for Ly alpha broadening by neutral helium, improving hydrogen abundance measurements in DBA white dwarfs.

Findings

Good agreement between H/He ratios from Ly alpha and H alpha after new modeling.

Interstellar absorption explains discrepancies in distant stars.

Enhanced line profile modeling resolves previous measurement inconsistencies.

Abstract

Traces of photospheric hydrogen are detected in at least half of all white dwarfs with helium-dominated atmospheres through the presence of H alpha in high-quality optical spectroscopy. Previous studies have noted significant discrepancies between the hydrogen abundances derived from H alpha and Ly alpha 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 alpha by neutral helium. When fitting Hubble Space Telescope COS spectroscopy of 17 DB white dwarfs using our new line profile calculations, we find good agreement between log(H/He) measured from Ly alpha and H alpha. Larger values of log(H/He) based on Ly alpha 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.