Magnetic Fields in Core Collapse Supernovae: Possibilities and Gaps

Spectropolarimetry of core collapse supernovae has shown that they are asymmetric and often, but not universally, bi-polar. The Type IIb SN 1993J and similar events showed large scatter in the Stokes parameter plane. SN 2002ap which showed very high photospheric velocities in early phases revealed that the dominant axes associated with hydrogen, with oxygen, and with calcium were all oriented substantially differently. Observational programs clearly have much more to teach us about the complexity of asymmetric supernovae and the physics involved in the asymmetry. Jet-induced supernova models give a typical jet/torus structure that is reminiscent of some objects like the Crab nebula, SN 1987A and perhaps Cas A. Jets, in turn, may arise from the intrinsic rotation and magnetic fields that are expected to accompany core collapse. We summarize the potential importance of the magneto-rotational instability for the core collapse problem and sketch some of the effects that large magnetic fields, ∼ 10 G, may have on the physics of the supernova explosion. Open issues in the problem of multi-dimensional magnetic core collapse are summarized and a critique is given of some recent MHD collapse calculations. A crucial piece of information that can inform the discussion of potential MHD effects even in the absence of the explicit inclusion of magnetic fields is to give sufficient information from a rotating collapse to at least crudely estimate the time-dependent saturation field according to the prescription va ∼ rΩ. Many studies of rotating collapse produce such information, but fail to present it explicitly.