Collisional effects in the blue wing of the Balmer-alpha line

TL;DR

This paper investigates the collisional effects on the blue wing of the Balmer-alpha line in white dwarf atmospheres, using new molecular data and a unified broadening theory applicable at high helium densities.

Contribution

It provides new accurate H-He potential energies and transition dipole moments, advancing the theoretical understanding of line broadening in dense stellar atmospheres.

Findings

Predicted asymmetrical line shapes in the Balmer-alpha line due to collisional effects.

Implications for interpreting white dwarf spectra with high helium densities.

Enhanced models for spectral line broadening at high densities.

Abstract

In order to investigate the near wing of the Lyman-alpha line, accurate line profile calculations and molecular data are both required due to the existence of a close line satellite responsible for its asymmetrical shape. Lyman-alpha lines observed with the Cosmic Origin Spectograph (COS) on the Hubble Space Telescope ( HST) show this peculiarity in the spectra of DBA and DA white dwarf stars. A similar asymmetrical shape in the blue wing can be predicted in the Balmer-alpha line of H perturbed by He and H atoms. In continuation with a very recent work on the Lyman-alpha line, where the n=2 potential energies and transition dipole moments from the ground state were determined, we present new accurate H-He potential energies and electronic transition dipole moments involving the molecular states correlated with H(n=3)+He and their transition dipole moments with the states correlated with H(n=2)+He. Those new data and existing molecular data for H(n=2,3)-H are used to provide a theoretical investigation of the collisional effects in the blue wing of the Balmer-alpha line of H perturbed by He and H atoms. We note the consequences for the Balmer-alpha line shape in the physical conditions found in the cool atmosphere of DZA white dwarfs where helium densities may be as high as 10^21 cm-3. This study is undertaken with a unified theory of spectral line broadening valid at very high helium densities.