On excess entropy and latent heat in crystallizing white dwarfs

Abstract

Based on the linear mixing approach, we calculate the latent heat for crystallizing fully-ionized ${}^{12}$C/${}^{16}$O and ${}^{16}$O/${}^{20}$Ne 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 analyze the latent heat of crystallizing eutectic ${}^{12}$C/${}^{22}$Ne 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 ${}^{22}$Ne distillation in cooling C/O/${}^{22}$Ne WD proposed as a solution for the ultramassive WD multi-Gyr cooling anomaly. Astrophysical implications of our findings for crystallizing WD are discussed.

5 pages, 2 figures. Letter to MNRAS, in press

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