On excess entropy and latent heat in crystallizing white dwarfs

Abstract

ABSTRACT Based on the linear-mixing approach, we calculate the latent heat for crystallizing fully ionized 12C/16O and 16O/20Ne mixtures in white dwarf (WD) cores for two different parametrizations of the corrections to the linear-mixing energies and with account of ion quantum effects. We report noticeable composition-dependent deviations of the excess entropy in both directions from the standard value of 0.77 per ion. Within the same framework, we evaluate the excess entropy and released or absorbed heat accompanying the exsolution process in solidified WD layers. The inclusion of this effect is shown to be important for reliable interpretation of WD cooling data. We also analyse the latent heat of crystallization for the eutectic 12C/22Ne mixture, where we find a qualitative dependence of both the phase diagram and the latent heat behaviour on ion quantum effects. This may be important for the model with 22Ne distillation in cooling C/O/22Ne WD proposed as a solution for the ultramassive WD multi-Gyr cooling anomaly. Astrophysical implications of our findings for crystallizing WD are discussed.