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.