THE INFLUENCE OF CRYSTALLIZATION ON THE LUMINOSITY FUNCTION OF WHITE-DWARFS

The inclusion of a detailed treatment of solidification processes in the cooling theory of carbon-oxygen white dwarfs is of crucial importance for the determination of their luminosity function. Carbon-oxygen separation at crystallization yields delays larger than 2 Gyr in the cooling time for a white dwarf to cool down to the observed cutoff luminosity, i.e., log (L/L.) approximately 4.5. When calculating the luminosity function, this leads to estimates of the age of the Galactic disk 1.5-2 Gyr older than the ones obtained when ignoring such separation processes. Furthermore, the presence of minor chemical species, in particular, Ne-22 and Fe-56, alters significantly the crystallization process and the cooling time and produces extra delays of 2-3 Gyr. However, the detailed computation of the theoretical white dwarf luminosity function, takng into account galactic chemical evolution, shows that, for minor species, this effect does not modify significantly the location of the cutoff, and thus the estimated age of the disk. The effect of Ne-22 crystallization is shown to produce a sharp peak in the luminosity function at log (L/L.) approximately - 3.8. Though nondetectable with present-day observations, this peak can be used as a future observational test of the crystallization of minor chemical species in white dwarf interiors.