Collisional effects in the blue wing of Lyman-α

F Spiegelman,J F Kielkopf,N F Allard

doi:10.1051/0004-6361/202140681

Abstract

Spectral observations below Lyman-α are now obtained with the Cosmic Origin Spectrograph (COS) on the Hubble Space Telescope. It is therefore necessary to provide an accurate treatment of the blue wing of the Lyman-α line that enables correct calculations of radiative transport in DA and DBA white dwarf stars. On the theoretical front, we very recently developed very accurate H-He potential energies for the hydrogen 1s, 2s, and 2p states. Nevertheless, an uncertainty remained about the asymptotic correlation of the Σ states and the electronic dipole transition moments. A similar difficulty occurred in our first calculations for the resonance broadening of hydrogen perturbed by collisions with neutral H atoms. The aim of this paper is twofold. First, we clarify the question of the asymptotic correlation of the Σ states, and we show that relativistic contributions, even very tiny, may need to be accounted for a correct long-range and asymptotic description of the states because of the specific 2s 2p Coulomb degeneracy in hydrogen. This effect of relativistic corrections, inducing small splitting of the 2s and 2p states of H, is shown to be important for the Σ-Σ transition dipole moments in H-He and is also discussed in H-H. Second, we use existent (H-H) and newly determined (H-He) accurate potentials and properties to provide a theoretical investigation of the collisional effects on the blue wing of the Lyman-α line of H perturbed by He and H. We study the relative contributions in the blue wing of the H and He atoms according to their relative densities. We finally achieve a comparison with recent COS observations and propose an assignment for a feature centered at 1190 Å.

Highlights

In helium-dominated white dwarfs, the discrepancy in the hydrogen abundance between Balmer-α from the optical data and Lyman-α from the ultraviolet (UV) data is strong
H-He potential energy curves (PECs) and correlation to dissociated atomic states For distances smaller than 8.1 Å, the results we show in Fig. 1 do not differ substantially from the recent calculation of Allard et al (2020), which was essentially achieved with a similar methodology
No explicit mention of the relativistic effects was made in Allard et al (2020), and the C state potential energy curve was found to converge to the unique Coulomb 2s/2p asymptote with a dipole moment for the X − C transition that vanished asymptotically, meaning an adiabatic correlation of the obtained C state with the dipole-forbidden 2s atomic state

Summary

Introduction

In helium-dominated white dwarfs, the discrepancy in the hydrogen abundance between Balmer-α from the optical data and Lyman-α from the ultraviolet (UV) data is strong. Relativistic effects that are smaller than 1 cm−1 for hydrogen are responsible for lifting the strict degeneracy of the hydrogen atomic levels in the Coulomb model (Kramida 2010) This level splitting is crucial for establishing the adiabatic correlation of the molecular states toward the asymptotic levels, and the specific asymptotic behavior of the dipole transition moments from the ground state. Triplet states only lead to a blue asymmetry because of a close line satellite that appears as a shoulder in the blue wing These improved theoretical calculations of the complete Lyman-α profile including both red and blue wings were applied to the interpretation of International Ultraviolet Explorer (IUE) and Hubble Space Telescope (HST) spectra.

Diatomic potentials and electronic transition dipole moments

Collisional profiles perturbed by neutral H and He atoms

C Σ 86000

Collisional profiles simultaneaously perturbed by He and H atoms

Observation of the 1150 Å satellite

Conclusions