Collisional effects in the blue wing of the Balmer-α line^★,★★

¹ GEPI, Observatoire de Paris, Université PSL, UMR 8111, CNRS, 61 avenue de l’Observatoire, 75014 Paris, France
e-mail: nicole.allard@obspm.fr
² Sorbonne Université, CNRS, UMR7095, Institut d’Astrophysique de Paris, 98bis Boulevard Arago, Paris, France
³ Laboratoire de Physique et Chimie Quantique, Fédération FERMI, Université de Toulouse (UPS) and CNRS, 118 route de Narbonne, 31400 Toulouse, France
⁴ Department of Physics and Astronomy, University of Louisville, Louisville, Kentucky 40292, USA

Received: 3 June 2021
Accepted: 20 September 2021

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.