Convection Destroys the Core/Mantle Structure in Hybrid C/O/Ne White Dwarfs

TL;DR

This study shows that hybrid C/O/Ne white dwarfs become unstable to rapid core/mantle mixing during cooling, which affects their composition and potential to produce Type Ia supernovae.

Contribution

It demonstrates that core/mantle mixing in hybrid white dwarfs occurs rapidly during cooling, challenging previous assumptions about their composition at the Chandrasekhar mass.

Findings

Hybrid WDs undergo significant core/mantle mixing within 2,000 years of cooling.

Mixing reduces the central carbon fraction, impacting supernova progenitor models.

Results are based on MESA simulations with different core sizes.

Abstract

A hybrid C/O/Ne white dwarf (WD) -- an unburned C/O core surrounded by an O/Ne/Na mantle -- can be formed if the carbon flame is quenched in a super-AGB (SAGB) star or white dwarf merger remnant. We show that this segregated hybrid structure becomes unstable to rapid mixing within 2,000 years of the onset of WD cooling. Carbon burning includes a weak reaction that removes electrons, resulting in a lower electron-to-baryon ratio ($Y_{\rm e}$) in the regions processed by carbon burning compared to the unburned C/O core, making the O/Ne mantle denser than the C/O core as the WD cools. This is unstable to efficient mixing. We use the results of $\texttt{MESA}$ models with different size C/O cores to quantify the rate at which the cores mix with the mantle as they cool. In all cases, we find that the WDs undergo significant core/mantle mixing on timescales shorter than the time available to grow the WD to the Chandrasekhar mass ($M_{\rm Ch}$) by accretion. As a result, hybrid WDs that reach $M_{\rm Ch}$ due to later accretion will have lower central carbon fractions than assumed thus far. We briefly discuss the implications of these results for the possibility of Type Ia supernovae from hybrid WDs.