| Abstract | A set of seven early spectra of SN 1993J in M81 taken between UT March 29.88 and UT April 5.02, when the supernova was within ̃6 days of shock breakout, is analyzed. The fact that they are well represented by the energy distribution of reddened blackbodies with color temperature decreasing in time is used in a multi- parametric fit to deconvolve self-consistent values for the foreground absorption, temperatures, and flux densities. The best-fitting absorption found is Av = 0.74±0.05 (standard deviation of the unweighted mean of five spectra). The measurements of the color temperature and flux densities of the supernova are used to study the evolution of the angular radius of its expanding photosphere. The evolution of the angular radius, combined with the measurement of the expansion velocity from spectral lines, is used to obtain constraints on the distance to M81, the size of the progenitor star at the time of explosion, and dynamics of early phase of expansion. The distance to M81 from the expansion parallax of SN 1993J is D = ζ(3.5±0.3) Mpc, where ζ ≤ 1 is the scattering parameter. The distance to M81 from the expanding photosphere method is D = ζ(3.48 -0.09δt0 + 0.09δR0± 0.20) Mpc, where δt0 is the difference between the actual time of shock breakout and JD = 2,449,074.6 in tenths of a day, and δR0 is the difference between the actual radius of the progenitor star and 3.86 × 1013 cm, in units of 1013 cm. In order to be consistent with the Cepheid distance, these estimates indicate that SN 1993J had ζ ̃1 at very early times, in contradiction with typical results for Type II supernovae. The radius of the progenitor star at the time of explosion must have been R0 ≳ 4.7 × 1013 cm = 675 Rsun. |
