Axisymmetric Circumstellar Interaction in Supernovae

Multiwavelength observations of Type II supernovae have shown evidence forthe interaction of supernovae with the dense slow winds from the red supergiantprogenitor stars. Observations of planetary nebulae and the nebula around SN1987A show that the slow winds from extended stars frequently have an axisymme-tric structure with a high density in the equatorial plane. We have carried outnumerical calculations of the interaction of a supernova with such an axisymme-tric density distribution. For small values of the angular density gradient atthe pole, the asymmetry in the interaction shell is greater than, but close to,that expected from purely radial motion. If the angular density gradient isabove a moderate value, the flow qualitatively changes and a protrusion emergesalong the axis. For a power-law supernova density profile, the flow approachesa self-similar state in which the protrusion length is $2-4$ times the radiusof the main shell. The critical density gradient is larger for steeper densityprofiles of the ejecta. Most of our calculations are axisymmetric, but we havecarried out a 3-dimensional calculation to show that the protrusion is not anumerical artifact along the symmetry axis. For typical supernova parameters,the protrusions take $\gtrsim$ several years to develop. The appearance of theshell with protrusions is similar to that observed in VLBI radio images of theremnant 41.9 +58 in M82 and, possibly, of SN 1986J. We also considered thepossibility of asymmetric ejecta and found that it had a relatively smalleffect on the asymmetry of the interaction region.Comment: 22 page postscript file (gzipped and uuencoded), 10 gzipped postscript figures may be retrieved from ftp://www.astro.su.se/pub/supernova/blc96_asym/ Submitted to Ap