  The spectra of helium-dominated white dwarf stars with hydrogen in their
atmosphere present a distinctive broad feature centered around 1160~\AA\/ in
the blue wing of the Lyman-$\alpha$ line. It is extremely apparent in WD
1425+540 recently observed with HST COS. With new theoretical line profiles
based on ab initio atomic interaction potentials we show that this feature is a
signature of a collision-induced satellite due to an asymptotically forbidden
transition. This quasi-molecular spectral satellite is crucial to understanding
the asymmetrical shape of Lyman-$\alpha$ seen in this and other white dwarf
spectra. Our previous work predicting this absorption feature was limited by
molecular potentials that were not adequate to follow the atomic interactions
with spectroscopic precision to the asymptotic limit of large separation. A new
set of potential energy curves and electronic dipole transition moments for the
lowest electronic states of the H--He system were developed to account
accurately for the behavior of the atomic interactions at all distances, from
the chemical regime within 1~\AA\/ out to where the radiating H atoms are not
significantly perturbed by their neighbors. We use a general unified theory of
collision-broadened atomic spectral lines to describe a rigorous treatment of
hydrogen Lyman-$\alpha$ with these potentials and present a new study of its
broadening by radiative collisions of hydrogen and neutral helium. These
results enable ab initio modeling of radiative transport in DBA white dwarf
atmospheres.
