SN 1993J VLBI. I. The Center of the Explosion and a Limit on Anisotropic Expansion

Phase-referenced VLBI observations of supernova 1993J at 24 epochs, from 50days after shock breakout to the present, allowed us to determine thecoordinates of the explosion center relative to the quasi-stationary core ofthe host galaxy M81 with an accuracy of 45 micro-arcsec, and to determine thenominal proper motion of the geometric center of the radio shell with anaccuracy of 9micro-arcsec/yr. The uncertainties correspond to 160 AU for theposition and 160 km/s for the proper motion at the distance of the source of3.63 Mpc. After correcting for the expected galactic proper motion of thesupernova around the core of M81 using HI rotation curves, we obtain a peculiarproper motion of the radio shell center of only 320 +/- 160 km/s to the south,which limits any possible one-sided expansion of the shell. We also find thatthe shell is highly circular, the outer contours in fact being circular towithin 3%. Combining our proper motion values with the degree of circularsymmetry, we find that the expansion of the shockfront from the explosioncenter is isotropic to within 5.5% in the plane of the sky. This is a morefundamental result on isotropic expansion than can be derived from thecircularity of the images alone. The brightness of the radio shell, however,varies along the ridge and systematically changes with time. The degree ofisotropy in the expansion of the shockfront contrasts with the asymmetries andpolarization found in optical spectral lines. Asymmetric density distributionsin the ejecta or more likely in the circumstellar medium, are favored toreconcile the radio and optical results. We see no sign of any disk-likedensity distribution of the circumstellar material, with the average axis ratioof the radio shell of SN1993J being less than 1.04.Comment: 21 pages, LaTex + 5 Figures (encapulsated PostScript), Accepted for Publication in the Astrophysical Journa