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

The inclusion of a detailed treatment of solidification processes in t he cooling theory of carbon-oxygen white dwarfs is of crucial importan ce for the determination of their luminosity function. Carbon-oxygen s eparation at crystallization yields delays larger than 2 Gyr in the co oling time for a white dwarf to cool down to the observed cutoff lumin osity, i.e., log (L/L.) approximately 4.5. When calculating the lumino sity 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 par ticular, Ne-22 and Fe-56, alters significantly the crystallization pro cess and the cooling time and produces extra delays of 2-3 Gyr. Howeve r, 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 locat ion 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 lumin osity function at log (L/L.) approximately - 3.8. Though nondetectable with present-day observations, this peak can be used as a future obse rvational test of the crystallization of minor chemical species in whi te dwarf interiors.