SN 1993J VLBI. IV. A Geometric Distance to M81 with the Expanding Shock Front Method

We compare the angular expansion velocities, determined with VLBI, with the linear expansion velocities measured from optical spectra for supernova 1993J in the galaxy M81, over the period from 7 days to ~9 yr after shock breakout, and estimate the distance to SN 1993J using the expanding shock front method (ESM). We find that the best distance estimate is obtained by fitting the angular velocity of a point halfway between the contact surface and outer shock front to the maximum observed hydrogen gas velocity. We obtain a direct, geometric, distance estimate for M81 of D = 3.96 ± 0.05 ± 0.29 Mpc with statistical and systematic error contributions, respectively, which combine to a total standard error of ±0.29 Mpc. The upper limit of 4.25 Mpc corresponds to the hydrogen gas with the highest observed velocity just reaching out to the contact surface a few days after shock breakout. The lower limit of 3.67 Mpc corresponds to this gas reaching as far out as the forward shock for the whole observing period, which would mean that Rayleigh-Taylor fingers have grown to the forward shock already a few days after shock breakout. Our distance estimate is 9% ± 13% larger than that of 3.63 ± 0.34 Mpc from the HST Key Project. The radio shell and the Hα absorbing and emitting gas are similarly decelerated on average, but the latter slightly less so than the former several years after shock breakout. This may indicate developing Rayleigh-Taylor fingers, extending progressively further into the shocked circumstellar medium.