Exploring the Carbon Simmering Phase: Reaction Rates, Mixing, and the Convective Urca Process

TL;DR

This paper investigates the role of mixing and the convective Urca process during the carbon simmering phase in Type Ia supernova progenitors, highlighting their impact on neutron excess and model discrepancies.

Contribution

It demonstrates the significance of the convective Urca process and mixing treatment in modeling the simmering phase, emphasizing the need for improved stellar evolution models.

Findings

Heating from weak reactions raises WD temperature.

The convective Urca process significantly influences simmering.

Model discrepancies relate to different net energetics of the Urca process.

Abstract

The neutron excess at the time of explosion provides a powerful discriminant among models of Type Ia supernovae. Recent calculations of the carbon simmering phase in single degenerate progenitors have disagreed about the final neutron excess. We find that the treatment of mixing in convection zones likely contributes to the difference. We demonstrate that in MESA models, heating from exothermic weak reactions plays a significant role in raising the temperature of the WD. This emphasizes the important role that the convective Urca process plays during simmering. We briefly summarize the shortcomings of current models during this phase. Ultimately, we do not pinpoint the difference between the results reported in the literature, but show that the results are consistent with different net energetics of the convective Urca process. This problem serves as an important motivation for the development of models of the convective Urca process suitable for incorporation into stellar evolution codes.