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

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 ($Y_{\mathrm{e}}$) 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 $\text{MESA}$ 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 ($M_{\mathrm{Ch}}$) by accretion. As a result, hybrid WDs that reach $M_{\mathrm{Ch}}$ 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.

6 pages, 5 figures, updated to correct author affiliations

References in corpus (8)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Deflagrations in hybrid CONe white dwarfs: a route to explain the faint Type Iax supernova 2008ha
• Chemical Transport and Spontaneous Layer Formation in Fingering Convection in Astrophysics
• Neutronization During Type Ia Supernova Simmering
• The Reduction of the Electron Abundance during the Pre-explosion Simmering in White Dwarf Supernovae
• On Carbon Burning in Super Asymptotic Giant Branch Stars
• Hybrid C-O-Ne white dwarfs as progenitors of type Ia supernovae: dependence on Urca process and mixing assumptions
• The hybrid CONe WD + He star scenario for the progenitors of type Ia supernovae

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