Collision-induced satellite in the blue wing of the Balmer-beta line and consequences on the Balmer series

TL;DR

This study investigates how collisions with helium atoms cause asymmetries and satellite features in the Balmer series lines of white dwarf stars, improving understanding of spectral line shapes at high helium densities.

Contribution

It provides new potential energy data and transition dipole moments for H-He molecular states, enabling detailed modeling of collisional effects on Balmer line profiles.

Findings

Collision-induced satellites explain spectral asymmetries.

Line core amplitude decreases rapidly with series order at high helium densities.

Unified theory accurately models line broadening in white dwarf atmospheres.

Abstract

In this paper we emphasize the non-Lorentzian behavior of the Balmer series in helium-dominated DBA white dwarf stars for which the decades-old problem exists for the determination of the hydrogen abundance. In a very recent work, we have shown that quasi-molecular line satellites due to H-He and H-H collisions are responsible for the asymmetrical shape of the Lyman-alpha lines observed with the Cosmic Origin Spectrograph (COS) and that a similar asymmetry exists for the Balmer-alpha line profiles. In continuation with very recent work, where the n=2, 3 potential energies and transition dipole moments from the ground state were determined, here, we present accurate H-He potential energies and electronic transition dipole moments concerning the molecular states correlated with H(n=4)+He and their transition dipole moments with the states correlated with H(n=2)+He. Those new data are used to provide a theoretical investigation of the collisional effects in the blue wing of the Balmer-beta line of H perturbed by He. Because of the general trend characterizing the repulsive Sigma states of the potential energies involved in the Balmer series, the amplitude in the core of the line is decreasing very fast with the order of the series when the helium density gets as large as 10^21 cm^-3. This study is undertaken by applying a unified theory of spectral line broadening that is valid at very high helium densities found in DZA white dwarf stars. The treatment includes collision-induced (CI) line satellites due to asymptotically forbidden transitions, and it explains the asymmetry observed in their spectra.