THE INTERNAL SHEAR OF TYPE Ia SUPERNOVA PROGENITORS DURING ACCRETION AND SIMMERING

Awhite dwarf (WD) gains substantial angular momentum during the accretion process that grows it toward a Chandrasekhar mass. It is therefore expected to be rotating quickly when it ignites as a Type Ia supernova. The thermal and shearing profiles are important for subsequent flame propagation. We highlight processes that could affect the WD shear during accretion, as well as during the 1000 yr 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 aWD substantially past the typical Chandrasekhar mass. Once carbon ignites, a convective region spreads from the center of the WD. 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, including a demonstration 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/nonconvective boundary. Subject headinggs: accretion, accretion disks — convection — hydrodynamics — stars: magnetic fields — white dwarfs