On the Importance of the Convective Urca Process in 3D Simulations of a Simmering White Dwarf

TL;DR

This study uses 3D simulations to investigate how the convective Urca process influences convection and energy loss in simmering white dwarfs, shedding light on the pre-supernova phase.

Contribution

First 3D simulations explicitly compare the effects of the convective Urca process on convection and neutrino losses in white dwarf simmering phases.

Findings

Convective Urca process reduces the size of the convection zone.

Convection still extends beyond the Urca shell despite the process.

Results can inform 1D stellar models of white dwarf evolution.

Abstract

Type Ia supernovae are bright thermonuclear explosions that are important to numerous areas of astronomy. However, the origins of these events are poorly understood. One proposed setting is that of a near Chandrasekhar mass white dwarf that undergoes runaway carbon burning in the core. During the thousand years leading up to the explosion, the white dwarf undergoes a simmering phase where slow carbon burning heats the core and drives convection. A poorly understood aspect of this phase is the convective Urca process, which links convection with weak nuclear reactions. We use the low Mach number code MAESTROeX to perform full 3D simulations as is required to accurately capture the turbulent convection. We present simulations with and without the A=23 convective Urca process, which have relaxed to a steady state. We characterize the effects of the convective Urca process on the neutrino losses, the nuclear energy generation, and the convective boundary. We find that the size of the convection zone is substantially reduced by the convective Urca process, though convection still extends past the Urca shell. Our findings on the structure of the convective zone and the compositional changes can be used to inform 1D stellar models that track the longer-timescale evolution.