2000A&A...361..283J

We have modified our version of the marcs stellar atmosphere code to make it possible to compute white dwarf atmospheres, and we have constructed and present here the necessary input data for such calculations. With this new tool we have computed a number of helium-rich white dwarf atmospheres, synthetic spectra, and broad band colours, and we compare them here with observational data from the literature. The major differences between our code and existing white dwarf codes in the literature, are (1) a detailed computation of the chemical equilibrium between all relevant ions, neutral atoms, and molecules, (2) a detailed line-by-line calculation of the molecular opacity, and (3) a detailed quantum mechanical description of the collision-induced absorption (CIA) processes. The molecular line data include a few hundred million spectral lines from CO, CH, CN, CS, C₂, C₃, C₂H, HCN, and C₂H₂. The sampling of the lines is done by the opacity sampling (OS) method. The CIA data are calculated from ab initio dipole and potential surfaces suitable for high temperatures.
Result. for the H₂-H₂ absorption coefficient have been presented elsewhere, while the H₂-He CIA data are obtained specifically for this study, and are presented at frequencies from 800 to 20000cm^–1 and at temperatures from 1000 to 7000K. The new data are available at www.astro.ku.dk/∼aborysow/programs/ . Our code is particularly useful for description of the cooler white dwarfs. These have received increased attention during recent years because of better observational facilities. They are important for understanding the conditions in the early phase of the evolution of our Galaxy and as a tool to determine the age of the Galactic disk and halo. We demonstrate the importance of including all elements (i.e. not only hydrogen and helium as is done in several existing models in the literature), and of including collision-induced opacities and molecular line opacities in the atmospheric models. Our results are in good agreement with existing photometry of cool white dwarfs. In particular, we can explain the scatter in observational colour-colour diagrams as a consequence of small variations in the C/O ratio. Finally we discuss, based on our new models, the possible role of molecular C₂H in so-called C₂H white dwarfs.