New Conductive Opacities for White Dwarf Envelopes

Abstract

Thanks to their continuous cooling and relative simplicity, white dwarf stars are routinely used to measure the ages of stellar populations. The usefulness of white dwarfs as cosmochronometers depends on the availability of accurate cooling models. A key ingredient of those models are the conductive opacities, which largely govern the cooling rate. In this work, we present improved conductive opacities for the regime of moderate coupling and moderate degeneracy that characterizes an important portion of the envelopes of DA and DB white dwarfs. We find differences of up to a factor 3 between our calculations and the commonly used opacities of Cassisi et al. (2007), which we attribute to an improved account of electron-electron scattering. The cooling models are strongly affected by those changes in the conductive opacities: the age of a 4000 K white dwarf can be reduced by as much as 2 Gyr. We provide analytical fits to our new opacities to facilitate the implementation of this important effect in white dwarf evolution codes.

11 pages, 6 figures, 3 tables. Accepted for publication in The Astrophysical Journal

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• Modeling contact binaries, II. The effect of energy transfer
• Thermal evolution of neutron stars in soft X-ray transients with thermodynamically consistent models of the accreted crust
• The HST large programme on NGC 6752 -- IV. The White Dwarf Sequence
• Cooling Models for the Most Massive White Dwarfs