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

N F Allard,F Spiegelman,J F Kielkopf,S Bourdreux

doi:10.1051/0004-6361/202141461

Abstract

In order to investigate the near wing of the Lyman-α 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-α lines observed with the Cosmic Origin Spectograph on the Hubble Space Telescope 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-α line of H perturbed by He and H atoms. In continuation with a very recent work on the Lyman-α 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-α line of H perturbed by He and H atoms. We note the consequences for the Balmer-α line shape in the physical conditions found in the cool atmosphere of DZA white dwarfs where helium densities may be as high as 1021 cm−3. This study is undertaken with a unified theory of spectral line broadening valid at very high helium densities.

Highlights

Accurate atomic and molecular data, both theoretical and experimental, are required to fully exploit current and future astrophysical space missions and ground-based facilities that deliver sensitive precise spectroscopy
There is a discrepancy in the hydrogen abundance determined from optical spectra of Balmer-α and the abundance determined from ultraviolet spectra of Lyman-α that is possibly due to calculated opacities for these lines that have been incorporated into stellar atmosphere code
We have noted previously that there is a reliance on simplified models of neutral atom collision line-broadening physics, some dating to the work of Unsold (1955), in most of the codes that are in current use

Summary

Introduction

Accurate atomic and molecular data, both theoretical and experimental, are required to fully exploit current and future astrophysical space missions and ground-based facilities that deliver sensitive precise spectroscopy. The calculations reported in Allard et al (2008) support the results of Barklem et al (2000a, 2002) that the Ali & Griem (1966) theory of atom-atom resonance broadening for hydrogen underestimates the actual line width This will be the topic of a forthcoming paper; the present work is focussed on the blue wing of the Balmer-α and its asymmetrical shape due to radiation during close collisions. Similar to that due to H-He. In a recent work, we investigated the absorption features in the blue wings by computing detailed collisional broadening profiles (Spiegelman et al 2021) for both H-He and H-H using new Multi-Reference Configuration Interaction (MRCI) calculations of the excited states potential energy curves of H-He dissociating into H(n = 2)+He, as well as the relevant electric dipole transition moments from the ground state contributing to the Lyman-α spectrum.

Diatomic H-He potentials and electronic transition dipole moments

Collisional profiles perturbed by He atoms

Collisional profiles perturbed by H atoms

Self-broadening of the Balmer-α line

Blue wing of the Balmer-α line

General trend of the repulsive Σ states and conclusion