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The continuous cooling of a white dwarf is punctuated by events that affectits cooling rate. Probably the most significant of those is the crystallizationof its core, a phase transition that occurs once the C/O interior has cooleddown below a critical temperature. This transition releases latent heat as wellas gravitational energy due to the redistribution of the C and O ions duringsolidification, thereby slowing down the evolution of the white dwarf. Theunambiguous observational signature of core crystallization - a pile-up ofobjects in the cooling sequence - was recently reported. However, existingevolution models struggle to quantitatively reproduce this signature, castingdoubt on their accuracy when used to measure the ages of stellar populations.The timing and amount of the energy released during crystallization depend onthe exact form of the C/O phase diagram. Using the advanced Gibbs-Duhemintegration method and state-of-the-art Monte Carlo simulations of the solidand liquid phases, we have obtained a very accurate version of this phasediagram, allowing a precise modeling of the phase transition. Despite thisimprovement, the magnitude of the crystallization pile-up remainsunderestimated by current evolution models. We conclude that latent heatrelease and O sedimentation alone are not sufficient to explain theobservations and that other unaccounted physical mechanisms, possibly $^{22}$Nephase separation, play an important role.

Toward Precision Cosmochronology: A New C/O Phase Diagram for White Dwarfs