THE SHAPE, EXPANSION RATE AND DISTANCE OF SUPERNOVA-1993J FROM VLBI MEASUREMENTS

SUPERNOVA 1993J is one of the closest supernovae discovered this centu ry, allowing observations with very high linear resolution. Radio emis sion from the supernova became visible within days of the first optica l peak, and has remained strong, making this an ideal source for very- long-baseline interferometry (VLBI) investigations1-7. Here we report the results of a series of VLBI observations, made from one to three m onths after the supernova explosion. We find that the supernova is cir cularly symmetric, which is somewhat surprising in view of suggestions that the progenitor was a member of a binary system8,9, and the asymm etry implied by optical observations10. The supernova shows no sign of deceleration, and the expansion velocities that we estimate from the VLBI measurements are consistent with the maximum optical line velocit ies11, suggesting that the radio emission indeed arises from the shock front, where the ejecta are hitting the gas that surrounded the proge nitor star. We combine the angular expansion rate determined by the VL BI data with the optically derived expansion speed to estimate a dista nce to M81 of 4.0 +/- 0.6 Mpc, consistent with the value obtained from measurements of Cepheid variables in M81, 3.63 +/- 0.34 Mpc (ref. 12) .