The Cooling of CO White Dwarfs: Influence of the Internal Chemical Distribution

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Abstract
White dwarfs are the remnants of stars of low and intermediate masses on the main sequence. Since they have exhausted all of their nuclear fuel, their evolution is just a gravothermal process. The release of energy only depends on the detailed internal structure and chemical composition and on the properties of the envelope equation of state and opacity; its consequences on the cooling curve (i.e., the luminosity vs. time relationship) depend on the luminosity at which this energy is released. The internal chemical profile depends on the rate of the 12C(α, γ)16O reaction as well as on the treatment of convection. High reaction rates produce white dwarfs with oxygen-rich cores surrounded by carbon-rich mantles. This reduces the available gravothermal energy and decreases the lifetime of white dwarfs. In this paper we compute detailed evolutionary models providing chemical profiles for white dwarfs having progenitors in the mass range from 1.0 to 7 M☉, and we examine the influence of such profiles in the cooling process. The influence of the process of separation of carbon and oxygen during crystallization is decreased as a consequence of the initial stratification, but it is still important and cannot be neglected. As an example, the best fit to the luminosity functions of Liebert et al. and Oswalt et al. gives an age of the disk of 9.3 and 11.0 Gyr, respectively, when this effect is taken into account, and only 8.3 and 10.0 Gyr when it is neglected.
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