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4. White Dwarf Atmospheres

Published online by Cambridge University Press:  25 April 2016

Rainer Wehrse

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The main characteristic of white dwarf atmospheres is a pressure that is at least two orders of magnitude higher than in main sequence star atmospheres of the same effective temperature. This is due to the high gravity (log g ≈ 8), extreme metal under-abundances (Δlog εM ≳ 2.5), and (in many cases) the replacement of hydrogen by helium as the main constituent. As consequences the atmospheres are very thin (ΔR ≲ a few km) and the level populations of all species are given by the Boltzmann distribution (perhaps with the exception of the extreme outer layers, Greenstein 1973, Pilachowski 1984). Thus, models can be calculated under the assumption of plane parallel radiative transfer and local thermodynamic equilibrium, which facilitates the numerics very much;but special care has to be taken of pressure effects (e.g., broadening of spectral lines, quenching of levels, changes in the dissociation-ionization equilibria). In addition, the proper consideration of convection, which is very effective and may reach into the optically thin layers, makes the construction of model atmospheres for white dwarfs rather tedious and costly.

Type
Research Article
Information
Transactions of the International Astronomical Union , Volume 19 , Issue 1 , 1985 , pp. 513 - 515
Copyright
Copyright © Reidel 1985

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