Toward precision cosmochronology

Abstract

The continuous cooling of a white dwarf is punctuated by events that affect its cooling rate. The most significant of these events is the crystallization of its core, a phase transition that occurs once the C/O interior has cooled down below a critical temperature. This transition releases latent heat, as well as gravitational energy due to the redistribution of the C and O ions during solidification, thereby slowing down the evolution of the white dwarf. The unambiguous observational signature of core crystallization–a pile-up of objects in the cooling sequence–was recently reported. However, existing evolution models struggle to quantitatively reproduce this signature, casting doubt on their accuracy when used to measure the ages of stellar populations. The timing and amount of the energy released during crystallization depend on the exact form of the C/O phase diagram. Using the advanced Gibbs–Duhem integration method and state-of-the-art Monte Carlo simulations of the solid and liquid phases, we obtained a very accurate version of this phase diagram that allows a precise modeling of the phase transition. Despite this improvement, the magnitude of the crystallization pile-up remains underestimated by current evolution models. We conclude that latent heat release and O sedimentation alone are not sufficient to explain the observations, and that other unaccounted physical mechanisms, possibly 22Ne phase separation, play an important role.

$^{22}$Ne Phase Separation As A Solution To The Ultramassive White Dwarf Cooling Anomaly

“…If a more standard composition is assumed (e.g., scenario (c), X(O) = 0.60 and X( 22 Ne) = 0.014, or x O = 0.53 and x Ne = 0.009), then no 22 Ne distillation can initially occur and crystallization takes place as in the case of a two-component C/O plasma without any significant change to the 22 Ne distribution. As the crystallization front progresses outward, the liquid is gradually depleted in O due to C/O phase separation and it freezes at increasingly lower temperatures (Horowitz et al 2010;Althaus et al 2012;Blouin et al 2020). Eventually, Γ C,m crosses the boundary that delimits the distillation regime and 22 Ne distillation can start (this corresponds to the tip of the orange arrow in Figure 3).…”

“…To obtain a high-resolution, high-precision version of the C/O/Ne phase diagram at small 22 Ne concentrations, we turn to the Clapeyron integration technique that we have recently developed to map the phase diagrams of dense plasmas (Blouin & Daligault 2021, under review) and applied to the two-component C/O plasma (Blouin et al 2020). Briefly, this method consists of directly integrating the liquid-solid coexistence line using the appropriate Clapeyron equation for phase transitions at constant temperature and pressure (here, Equation A2 of Blouin & Daligault 2021), which we evaluate using Monte Carlo simulations where the full electron-ion plasma is considered.…”

The White Dwarfs of the Old, Solar Metallicity Open Star Cluster Messier 67: Properties and Progenitors

“…In addition to the extremely narrow peak at 0.59 M , the field WD mass distribution also shows an over-abundance of 0.7 − 0.9 M WDs below T eff = 10, 000 K due to a delay in their cooling from the release of latent heat of crystallization and related effects (e.g. Bauer et al 2020;Blouin et al 2020;Caplan et al 2020). Given the close agreement with the M67 WD masses, we conclude that the majority of local field WDs originated from M = 1.5 M main-sequence stars.…”

Precise Diffusion Coefficients for White Dwarf Astrophysics

“…These WDs may be the products of double WD mergers, which could produce the inferred excess of 22 Ne (Cheng 2019;Camisassa et al 2020). These recent successes have driven theoretical efforts to improve inputs to those models; for example, a new C/O phase diagram has been reported by Blouin et al 2020 and crystallization of C/O/Ne (and C/O/Fe) mixtures has been revisited with MD (Caplan et al 2020). Notably, the abundance of 22 Ne inferred by modelers is ultimately dependent on diffusion coefficients 𝐷 which set cooling timescales (Bildsten & Hall 2001;Bauer et al 2020).…”