Lyman-alpha wing absorption in cool white dwarf stars

Abstract

Kowalski & Saumon (2006) identified the missing absorption mechanism in the observed spectra of cool white dwarf stars as the Ly-alpha 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-alpha 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 of 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-alpha 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 Ly-alpha 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 broadband colour indices for model atmospheres with Teff<5000 K. These results confirm those previously obtained by Kowalski & Saumon (2006). 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.

11 pages, 12 figures, 1 table, to be published in MNRAS

References in corpus (6)

• Evolutionary and pulsational properties of white dwarf stars
• Found: The missing blue opacity in atmosphere models of cool hydrogen white dwarfs
• New cooling sequences for old white dwarfs
• White dwarf atmosphere models with Ly-$\alpha$ opacity in the analysis of the white dwarf cooling sequence of NGC 6397
• The Pseudo-continuum Bound-free Opacity of Hydrogen and its Importance in Cool White Dwarf Atmospheres
• Thermo-statistical description of gas mixtures from space partitions

Cited by in corpus (17)

• The 40 pc sample of white dwarfs from Gaia
• New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution
• Outer boundary conditions for evolving cool white dwarfs
• Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts
• The cool magnetic DAZ white dwarf NLTT 10480
• 11-12 Gyr old White Dwarfs 30 parsecs Away
• Cool white dwarf companions to four millisecond pulsars
• Discovery of near-ultraviolet counterparts to millisecond pulsars in the globular cluster 47 Tucanae
• VLT/X-shooter observations and the chemical composition of cool white dwarfs
• A Bayesian Approach to Deriving Ages of Individual Field White Dwarfs
• Convection and rotation boosted prescription of magnetic braking: application to the formation of extremely low-mass white dwarfs
• The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution
• Near-UV absorption in very cool DA white dwarfs
• Theoretical study of the line profiles of the hydrogen perturbed by collisions with protons
• Photometric White Dwarf Rotation
• Diagnosing pulsar winds in black-widow, redback, and other binary millisecond pulsar systems
• Unified line profiles for hydrogen perturbed by collisions with protons: satellites and asymmetries