Lyman α wing absorption in cool white dwarf stars

Abstract

Kowalski & Saumon identified the missing absorption mechanism in the observed spectra of cool white dwarf stars as the Lyman α red wing formed by the collisions between atomic and molecular hydrogen and successfully explained entire spectra of many cool DA‐type white dwarfs. Owing to the important astrophysical implications of this issue, we present here an independent assessment of the process. For this purpose, we compute free–free quasi‐molecular absorption in Lyman α due to collisions with H and H2 within the one‐perturber, quasi‐static approximation. Line cross‐sections are obtained using theoretical molecular potentials to describe the interaction between the radiating atom and the perturber. The variation in the electric dipole transition moment with the interparticle distance is also considered. Six and two allowed electric dipole transitions due to H–H and H–H2 collisions, respectively, are taken into account. The new theoretical Lyman α line profiles are then incorporated in our stellar atmosphere program for the computation of synthetic spectra and colours of DA‐type white dwarfs. Illustrative model atmospheres and spectral energy distributions are computed, which show that Lyman α broadening by atoms and molecules has a significant effect on the white dwarf atmosphere models. The inclusion of this collision‐induced opacity significantly reddens spectral energy distributions and affects the broad‐band colour indices for model atmospheres with Teff < 5000 K. These results confirm those previously obtained by Kowalski & Saumon. Our study points out the need for reliable evaluations of H3 potential energy surfaces covering a large region of nuclear configurations, in order to obtain a better description of H–H2 collisions and a more accurate evaluation of their influence on the spectrum of cool white dwarfs.

New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution

“…To account for this, we considered radiative opacities tables from OPAL for arbitrary metallicities. For effective temperatures less than 10 000 K, outer boundary conditions for the evolving models are given by detailed non-gray model atmospheres that incorporate non-ideal effects in the gas equation of state and chemical equilibrium (based on the occupation probability formalism), radiative and convective transport (mixing length theory) of energy, collision-induced absorption from molecules, and the Lyα quasi-molecular opacity (Rohrmann et al 2011). This provides detailed and accurate outer boundary conditions which are required for a proper treatment of the evolutionary behavior of cool white dwarfs.…”

Outer boundary conditions for evolving cool white dwarfs

“…5 for log ρ = −8 and −1. While the Lyα opacity affects the energy distribution emitted by white dwarfs cooler than T eff ≈ 5000 K (Kowalski & Saumon 2006;Rohrmann et al 2011), it has a moderate effect on the Rosseland opacity at low densities and relatively high temperatures (see Fig. 5), when the convective coupling with stellar core has not yet occurred.…”

“…The microphysics comprises nonideal effects in the gas equation of state and chemical equilibrium based on the occupation probability formalism describe in Rohrmann et al (2002 (Jorgensen et al 2000), and H-He pairs (Gustafsson & Frommhold 2001) were also taken into account. Model atmospheres explicitly include the Lyman α quasi-molecular opacity as a result of eight allowed electric dipole transitions arising from H-H and H-H 2 collisions (Rohrmann et al 2011). This opacity reduces the predicted flux at wavelength λ < 4000 Å for stars cooler than T eff ≈ 6000 K.…”

“…In our calculations, the outer boundary conditions were obtained using the pure-hydrogen LTE model atmospheres described at length in Rohrmann et al (2001Rohrmann et al ( , 2002Rohrmann et al ( , 2011. Specifically, we computed pressure, temperature, and outer mass fraction at a Rosseland mean optical depth τ Ross = 25.1189 (log τ Ross = 1.4) for 40 000 K ≤ T eff ≤ 2000 K and 6.5 ≤ log g ≤ 9.5.…”

“…The importance of using detailed boundary conditions was first addressed by Hansen (1998Hansen ( , 1999. Over the years, detailed model atmospheres have been developed that include a complete treatment of energy absorption processes such as collision-induced opacity (Bergeron et al 1991;Saumon & Jacobson 1999;Rohrmann 2001) and the Lyman α wing absorption (Kowalski & Saumon 2006;Rohrmann et al 2011). A&A 546, A119 (2012) Here we provide detailed boundary conditions that allow consistently computation of the evolution of cool white dwarfs with pure hydrogen atmospheres.…”