Collisional effects in the far red wing of Lyman-

¹ Observatoire de Paris, GEPI, UMR 8111, CNRS, 61 avenue de l'Observatoire, 75014 Paris, France e-mail: nicole.allard@obspm.fr
² Institut d'Astrophysique de Paris, UMR 7095, CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
³ Department of Physics and Astronomy, University of Louisville, Louisville, KY, 40292, USA

Received: 30 May 2008
Accepted: 14 October 2008

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 singlet transition 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.