Asymmetric Supernovae from Magnetocentrifugal Jets

Abstract

Strong toroidal magnetic fields generated in stellar collapse can generate magnetocentrifugal jets in analogy to those found in simulations of black hole accretion. Magnetocentrifugal jets may explain why all core collapse supernovae are found to be substantially asymmetric and predominantly bipolar. We describe two phases: the initial LeBlanc-Wilson jet and a subsequent protopulsar or toroidal jet that propagates at about the core escape velocity. The prompt LeBlanc-Wilson jets will produce an excess of neutron-rich matter and hence cannot be the common origin of supernova explosions; similar but less severe problems arise with the protopulsar jet that may be alleviated by partial evacuation along the axis by rotation. The jets will produce bow shocks that tend to expel matter, including iron and silicon, into equatorial tori. This may help to account for observations of the element distribution in Cas A. A magnetic ``switch'' mechanism may apply in rare instances (when there is low density and large magnetic field), with subsequent increase in the speed and collimation of the toroidal jet. The conditions that turn the magnetic switch ``on'' would yield a jet that propagates rapidly and with small opening angle through the star, depositing relatively little momentum. The result could be enough infall to form a black hole. A third, highly relativistic jet from the rotating black hole could catch up to the protopulsar jet after it has emerged from the star. The interaction of these two jets plausibly could be the origin of the internal shocks thought to produce γ-ray bursts and could explain the presence of iron lines in the afterglow. Recent estimates that typical γ-ray burst energy is ~3×1050 ergs imply either a very low efficiency for conversion of rotation into jets by the Blandford-Znajek mechanism or a rather rapid turnoff of the jet process even though the black hole still rotates rapidly. Magnetars and ``hypernovae'' might arise in an intermediate parameter regime of energetic jets that yield larger magnetic fields and provide more energy than the routine case, but that are not so tightly collimated that they yield failed supernova.