Collisional effects in the far red wing of Lyman-$\sf \alpha$

Abstract

Context. An accurate determination of the line broadening of the Lyman series of atomic hydrogen has been shown to be fundamental to interpretating UV and FUV spectra of DA white dwarfs. Quasi-molecular lines have been detected in the red wing of Lyman-α, Lyman-β, and Lyman-γ. They arise from radiative collisions of excited atomic hydrogen with unexcited neutral hydrogen atoms or protons. Aims. The aim of this paper is twofold. First, we examine the range of validity of the one-perturber approximation widely used to calculate the line wing. Second, we study the relative contributions of the two main transitions contributing to the far wing of the Lyman-α line profile according to the effective temperature and perturber density. Methods. In cool white dwarfs, the perturber density is so high that the one-perturber approximation breaks down and the collisional effects must be treated by using the autocorrelation formalism in order to take into account simultaneous collisions with more than one perturbing atom. Results. We show that, at the low temperatures of cool white dwarfs, the contribution of the singlettransition cannot be neglected in the calculation of Lyman-α profile perturbed by neutral hydrogen. A comparison with experimental laboratory spectra shows that the effects of multiple H-perturbers appear in the far wing. Conclusions. A reliable determination of the line profiles for the physical conditions of cool white dwarfs requires a unified theory that takes account of both the singlet and triplet transitions contributing to Lyman-α using accurate interaction potentials and radiative dipole transition moments. Multiple perturber effects must be considered and the autocorrelation formalism permits calculations for the densities of the atmospheres of cool white dwarfs.

Communication: Hartree-Fock description of excited states of H2

“…By propagating in both directions, we were able to obtain both symmetric and broken-symmetry solutions for several of the states. The Allard-Kielkopf correlation diagram 17 was used to check the dissociation products. Fig.…”

Lyman α wing absorption in cool white dwarf stars

“…The curves for b − i and b − h transitions are visually identical. Allard & Kielkopf (2009) noted in a recent study that simultaneous collisions with more than one perturber give an additional absorption from X − B transitions. Specifically, collisions with multiple atoms yield X − B opacity contributions, which dominate the 180-300 nm region.…”

Near-Uv Absorption in Very Cool Da White Dwarfs

“…This tension between the Lyα line wing and the CIA absorption indicates that one or perhaps both mechanisms are responsible for the mismatch. For example, the static broadening model of Kowalski & Saumon (2006) could be improved by considering dynamic collisions (Allard & Kielkopf 2009) or more accurate potential and dipole surfaces for the H-H 2 system during a collision. Another possibility is the corresponding "Lyα" absorption from a H 2 molecule between its electronic ground state and its first electronic excited state, which is not accounted for in the Kowalski & Saumon (2006) model.…”

“…Collisions of neutral H atoms with H, He and H 2 were modeled and Kowalski & Saumon (2006) showed that in very cool white dwarfs the broadening of Lyα was caused primarily by collisions with H 2 molecules. Subsequently, Allard & Kielkopf (2009) computed Lyα profiles broadened by collisions with H and He but not H 2 . As a result of this omission, their profiles do not extend into the optical and are unable to fully explain the flux deficiency observed in very cool WDs.…”