Progenitor Evolution and Chemical Stratification in CO White Dwarfs

Abstract

The potential of White Dwarfs (WDs) as a tool for deriving ages of various galactic components has been largely discussed in the literature (see the latest review by Fontaine et al. 2001 and references therein). Some important results have been obtained in dating old Open Clusters (e.g. Von Hippel et al 2000). A successful application to Globular Clusters could be similarly obtained in the next future, perhaps after few technological improvements (see e.g. Hansen et al. 2002). In any case, a reliable evaluation of the cooling timescale is required. Prada Moroni & Straniero (2002) have investigated various uncertainties affecting the calculation of WD cooling sequences, namely those related with nuclear reaction rates, convection, mass loss, initial chemical composition, opacity, equation of state and microscopic diffusion. They conclude that at the faint end of the cooling sequence (logL/L ⊙ ∼ -5.5) the present overall uncertainty on the age is of the order of 20%, which corresponds to about 3 Gyr. One of the main source of this uncertainty is related to the electron conductivity, which dominates the energy transport in WD interiors. The situation is illustrated in Fig. 1, where we report some theoretical cooling sequences of 0.6 M⊙ DA WDs, as obtained under different assumptions for the conductive opacity.