Extracting rotational energy in supernova progenitors: Transient Poynting flux growth vs. turbulent dissipation

Abstract

Observational evidence for anisotropy in supernovae (SN) may signal the importance of angular momentum and differential rotation in the progenitors. Free energy in differential rotation and rotation can be extracted magnetically or via turbulent dissipation. The importance that magnetohydrodynamic jets and coronae may play in driving SN motivates understanding large scale dynamos in SN progenitors. We develop a dynamical large scale interface dynamo model in which the differential rotation and rotation deplete both through Poynting flux and turbulent diffusion. We apply the model to a differentially rotating core surrounded by a convection zone of a SN progenitor from a initial 15 M ⊙ star. Unlike the Sun, the dynamo is transient because the differential rotation is primarily due to the initial collapse. Up to ∼10 51 erg can be drained into time-integrated Poynting flux and heat, the relative fraction of which depends on the relative amount of turbulence in the shear layer vs. the convection zone and the fraction of the shear layer into which the magnetic field penetrates. Both sinks can help facilitate explosions and could lead to different levels of anisotropy and pulsar kicks. In all cases, the poloidal magnetic field is much weaker than the toroidal field, and the Poynting flux is lower than previous estimates which invoke the magnitude of the total magnetic energy. A signature of a large scale dynamo is the oscillation of the associated Poynting flux on ∼1 s time scales, implying the same for the energy delivery to a SN.