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S. Akiyama, J.C. Wheeler (The University of Texas at Austin), D.L. Meier (Jet Propulsion Labolatory), I. Lichtenstadt (Hebrew University)
We investigate the effects of the magnetorotational instability (MRI) on collapsing, rotating iron cores. We assume that a weak seed field is exponentially amplified with a growth time t ~ 1/\Omega and sustained by MRI dynamo action. This process should dominate the linear growth of field due to winding of an initial seed field. We examine a variety of initial rotational profiles applied to the core of a 15 M\odot star. We follow the collapse with a one-dimensional flux-limited diffusion numerical code. The assumption that the specific angular momentum is conserved then yields an estimate of the angular velocity profile in the collapsed core that undergoes strong differential rotation. Distortion due to rotation and pressure and torques due to the magnetic field are neglected in this preliminary study. The magnetic field attains a strength of 1014-1016G within 50 msec after bounce, and peaks at the boundary of the proto-neutron star where the shear is the strongest. The ratio of magnetic pressure (Pm) to gas pressure (Pg) is \lesssim a few %, substantially below the equipartition value, for initial rotational frequency \Omega0 \lesssim 1 s-1. The resulting characteristic MHD luminosity is (LMHD ~ B2R3\Omega/2 ~1051-1054 erg s-1. Lcrit, the luminosity required for plasma to escape the gravitational potential, decreases with radius. Both Pm/Pg and LMHD/Lcrit peak just inside the stalled shock with initial solid-body rotation and at the boundary of the proto-neutron star with initial differential rotation. The rapid growth of the magnetic field may promote the formation of MHD jets up the rotation axis and, ultimately, a supernova explosion. This research is supported by NASA Grant NAG5-10766.
The author(s) of this abstract have provided an email address for comments about the abstract: shizuka@astro.as.utexas.edu