Mixing via Thermocompositional Convection in Hybrid C/O/Ne White Dwarfs

TL;DR

This study uses direct numerical simulations to investigate the mixing process in hybrid white dwarfs, confirming that they should be fully mixed before supernova explosions, which impacts understanding of their progenitors.

Contribution

First direct numerical simulations of core-mantle mixing in hybrid white dwarfs, validating previous stellar evolution approximations and emphasizing full mixing before supernovae.

Findings

Simulations show similar mixed region size as stellar models.

Hybrid white dwarfs are likely fully mixed before explosion.

Supports their role as progenitors of peculiar supernovae.

Abstract

Convective overshooting in super asymptotic giant branch stars has been suggested to lead to the formation of hybrid white dwarfs with carbon-oxygen cores and oxygen-neon mantles. As the white dwarf cools, this core-mantle configuration becomes convectively unstable and should mix. This mixing has been previously studied using stellar evolution calculations, but these made the approximation that convection did not affect the temperature profile of the mixed region. In this work, we perform direct numerical simulations of an idealized problem representing the core-mantle interface of the hybrid white dwarf. We demonstrate that, while the resulting structure within the convection zone is somewhat different than what is assumed in the stellar evolution calculations, the two approaches yield similar results for the size and growth of the mixed region. These hybrid white dwarfs have been invoked as progenitors of various peculiar thermonuclear supernovae. This lends further support to the idea that if these hybrid white dwarfs form then they should be fully mixed by the time of explosion. These effects should be included in the progenitor evolution in order to more accurately characterize the signatures of these events.