DECELERATION IN THE EXPANSION OF SN 1993J

A rarity among supernova, SN 1993J in M81 can be studied with high spa tial resolution. Its radio power and distance permit VLBI observations to monitor the expansion of its angular structure. This radio structu re was previously revealed to be shell-like and to be undergoing a sel f-similar expansion at a constant rate. From VLBI observations at wave lengths of 3.6 and 6 cm in the period 6-42 months after explosion, we have discovered that the expansion is decelerating. Our measurement of this deceleration yields estimates of the density profiles of the sup ernova ejecta and circumstellar material in standard supernova explosi on models.