Sn 1993j Vlbi (iv): a Geometric Determination of the 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 d to ∼9 yr after shock breakout. The high degree of isotropy of the radio shell’s expansion, within 5.5%, with the projection of the radio shell being circular within even 1.4%, and the consistency of the radio shell thickness with predictions from hydrodynamic simulations and analytical computations argue strongly in favor of the radio-emitting shell being bound by the forward and reverse shocks. The absorption and emission of hydrogen at the highest velocity most likely also arise between the forward and reverse shocks, specifically in the shocked ejecta behind the contact surface which is close to the reverse shock, and possibly in Rayleigh-Taylor fingers that extend beyond it, farther out into the radio shell but not beyond the forward shock. The radio shell and the Hα absorbing and emitting gas are on average similarly decelerated, 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. We estimate the distance to SN 1993J using the Expanding Shock Front Method (ESM). We find 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, corresponding 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 reaching no farther out than the contact surface a few days after shock breakout. The lower limit of 3.67 Mpc corresponds to this hydrogen gas reaching as far out as the forward shock for the whole 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, which is near our lower limit but within the errors. Subject headings: supernovae: individual (SN 1993J) — radio continuum: supernovae