Presentation #119.07 in the session “Evolved and Variable Stars”.
White dwarfs are burned-out stars condemned to a slow cooling that extends over billions of years. Because of this simple evolution, it is relatively easy to measure their ages, making them useful cosmic clocks to study the history of our Galaxy. Crucial to this age-dating technique are the cooling models used to translate white dwarf atmospheric parameters into cooling times. Thanks to Gaia DR2, we can now test in unprecedented detail the predictions of those cooling models. Important discrepancies between models and observations have surfaced over the last two years, prompting the need to review the constitutive physics of white dwarf evolution models. Using state-of-the-art computational techniques, we have revisited two key aspects of white dwarf cooling: (1) the phase separation of C and O during the crystallization of the core and (2) the conductivity of the H/He envelope. While this improved physics markedly affects the cooling models, disagreements with the observational data persist. The still uncertain role played by 22Ne has emerged as the number one suspect for those remaining discrepancies.