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 50 days after shock breakout to the present, allowed us to determine the coordinates of the explosion center relative to the quasistationary core of the host galaxy M81 with an accuracy of 45 mu as and to determine the nominal proper motion of the geometric center of the radio shell with an accuracy of 9 mu as yr(-1). The uncertainties correspond to 160 AU for the position and 160 km s(-1) for the proper motion at the distance of the source of 3.63 Mpc. After correcting for the expected galactic proper motion of the supernova around the core of M81 using H I rotation curves, we obtain a peculiar proper motion of the radio shell center of only 320 +/- 160 km s(-1) to the, south, which limits any possible one-sided expansion of the shell. We also find that the, shell is highly circular, the outer contours in fact being circular to within 3%. Combining our proper-motion values with the degree of circular symmetry, we find that the expansion of the shock front from the explosion center is isotropic to within 5.5% in the plane of the sky. This is a more fundamental result on isotropic expansion than can be derived from the circularity of the images alone. The brightness of the radio shell, however, varies along the ridge and systematically changes with time. The degree of isotropy in the expansion of the shock front contrasts with the asymmetries and polarization found in optical spectral lines. Asymmetric density distributions in the ejecta, or more likely in the circumstellar medium, are favored to reconcile the radio and optical results. We see no sign of any disklike density distribution of the circumstellar material, with the average axis ratio of the radio shell of SN 1993J being less than 1.04.