Three-component Phase Separation for Ultramassive White Dwarf Models

TL;DR

This study enhances white dwarf models by incorporating three-component phase separation, revealing significant impacts on chemical evolution, convection regimes, and pulsation properties, with implications for understanding their cooling and internal structure.

Contribution

We implement ternary phase diagrams into stellar evolution codes to model complex phase separation in ultramassive white dwarfs, extending beyond the traditional two-species approach.

Findings

Three-component phase separation significantly alters chemical profiles.

Fast overturning convection lasts up to 100 times longer with three species.

Cooling delay due to phase separation can reach up to 1 Gyr.

Abstract

We investigate phase separation in oxygen-neon (O/Ne) ultramassive white dwarfs (UMWDs). Current stellar evolution codes, such as MESA, only account for $\mathrm{^{16}O/^{20}Ne}$ separation and do not include other minor species. To improve this, we implement ternary phase diagrams into MESA. We construct UMWD models with O/Ne/sodium (Na) and O/Ne/magnesium (Mg) cores to test our implementation. We also assess the effect of including $\mathrm{^{22}Ne}$ in the current two-species framework. Our results show that incorporating additional components into the phase separation significantly alters the chemical evolution of UMWDs. Heavier elements preferentially enrich the solid core, enhancing mixing in the overlying liquid. We compute the buoyancy flux driven by compositional instabilities during crystallization. As in previous studies, we find two convective regimes: an early, fast overturning convection, lasting less than a million years, followed by inefficient (thermohaline) convection. The fast convective regime lasts up to 100 times longer with three-species separation compared to the standard $\mathrm{^{16}O/^{20}Ne}$ case. We find that neutron-rich species can have a significant contribution to the buoyancy flux despite their small mass fraction (<10\%). We compute the amount of cooling delay induced by phase separation in UMWDs, and find that the three-species phase separation produces a delay up to $\sim10$ times larger than the simplest case of fractionation, although still less than 1 Gyr. We predict that the change in the composition profile in the liquid region when three components are included should change the frequency of gravity modes that can propagate in the interior of pulsating UMWDs.