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

The Astrophysical Journal, Volume 876, Issue 1, article id. 10, 9 pp.

Josiah Schwab and Pascale Garaud

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.

The Deaths and Afterlives of Stars

I’m in Baltimore, MD at the Space Telescope Science Institute for the 2019 Spring Symposium.

Hubble Fellows Symposium

I’m in Baltimore, MD at the Space Telescope Science Institute for the 2019 Hubble Fellows Symposium. I’m giving a talk entitled Double White Dwarf Mergers and the Formation of R CrB Stars.

MESA Instrument Paper V

I’m in Santa Barbara, CA finalizing the fifth MESA Instrument paper. Here were all are, hard at work. (Photo taken by Matteo Cantiello.)

Residual Carbon in Oxygen–Neon White Dwarfs and Its Implications for Accretion-induced Collapse

The Astrophysical Journal, Volume 872, Issue 2, article id. 131, 11 pp.

Josiah Schwab and Kyle Akira Rocha

We explore the effects of the residual ¹²C present in oxygen-neon white dwarfs (ONe WDs) on their evolution toward accretion-induced collapse (AIC). We produce a set of ONe WD models using Modules for Experiments in Stellar Astrophysics (MESA) and illustrate how the amount and location of the residual carbon depends on the initial mass of the star and assumptions about rotation and convective overshooting. We find a wide range of possible ¹²C mass fractions roughly ranging from 0.1% to 10%. Convection and thermohaline mixing that occurs as the ONe WDs cool leads to nearly homogeneous interior compositions by the time that AIC would occur. We evolve these ONe WD models and some toy WD models toward AIC and find that regardless of the carbon fraction, the occurrence of Urca-process cooling due to ²³Na implies that the models are unlikely to reach carbon ignition before electron captures on ²⁴Mg occur. Difficulties associated with modeling electron-capture-driven convective regions in these ONe WDs prevent us from evolving our MESA models all the way to thermonuclear oxygen ignition and the onset of collapse. Thus, firm conclusions about the effect of carbon on the final fates of these objects await improved modeling. However, it is clear that the inclusion of residual carbon can shift the evolution from that previously described in the literature and should be included in future models.