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Astron. Astrophys. 361, 283-292 (2000)

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The atmospheres of cool, helium-rich white dwarfs

U.G. Jorgensen 1, D. Hammer 2, A. Borysow *  1 and J. Falkesgaard 1

1 Niels Bohr Institute, Astronomical Observatory, Juliane Maries vej 30, 2100 Copenhagen, Denmark
2 Department of Physics, The University of Texas at Austin, Austin, TX 78712, USA

Received 10 May 2000 / Accepted 16 June 2000


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. Results 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 20 000 cm-1 and at temperatures from 1 000 to 7 000 K. The new data are available at www.astro.ku.dk/[FORMULA]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.

Key words: stars: white dwarfs – molecular data – stars: atmospheres – infrared: stars

* On professional leave of absence from Physics Department, Michigan Technological University, Houghton, MI, 49931, USA

Send offprint requests to: U.G. Jorgensen (uffegj@nbi.dk)

© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000