The Internal Shear of Type Ia Supernova Progenitors During Accretion and Simmering

TL;DR

This paper investigates the internal shear dynamics of white dwarf progenitors during accretion and simmering phases, highlighting processes that influence rotation and convection, which are crucial for understanding Type Ia supernova ignition.

Contribution

It provides new insights into how baroclinic instabilities and magnetic fields affect white dwarf shear and rotation during accretion and simmering, impacting supernova progenitor models.

Findings

White dwarfs are likely to be nearly solid body rotators during accretion.

Shear growth is limited, constraining mass increase beyond Chandrasekhar limit.

Convection properties depend mainly on temperature or density ratios at the convection zone boundaries.

Abstract

A white dwarf (WD) gains substantial angular momentum during the accretion process that grows it toward a Chandrasekhar mass. It is therefore expected to be quickly rotating when it ignites as a Type Ia supernova. The thermal and shearing profile are important for subsequent flame propagation. We highlight processes that could affect the WD shear, during accretion as well as during the ~1000 years of pre-explosive simmering. Baroclinic instabilities and/or the shear growth of small magnetic fields provide sufficient torque to bring the WD very close to solid body rotation during accretion. The lack of significant shear makes it difficult to grow a WD substantially past the typical Chandrasekhar mass. Once carbon ignites, a convective region spreads from the WD's center. This phase occurs regardless of progenitor scenario, and therefore it is of great interest for understanding how the WD interior is prepared before the explosive burning begins. We summarize some of the key properties of the convective region, which includes demonstrating that the mass enclosed by convection at any given time depends most sensitively on a single parameter that can be expressed as either the ratio of temperatures or densities at the top and bottom of the convection zone. At low Rossby numbers the redistribution of angular momentum by convection may result in significant shearing at the convective/non-convective boundary.