SPECTROPHOTOMETRIC STUDY OF SN-1993J AT FIRST MAXIMUM LIGHT

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 similar to 6 d ays of shock breakout, is analyzed. The fact that they are well repres ented by the energy distribution of reddened blackbodies with color te mperature decreasing in time is used in a multiparametric fit to decon volve self-consistent values for the foreground absorption, temperatur es, and flux densities. The best-fitting absorption found is A(V) = 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 it s expanding photosphere. The evolution of the angular radius, combined with the measurement of the expansion velocity from spectral lines, i s used to obtain constraints on the distance to M81, the size of the p rogenitor star at the time of explosion, and dynamics of early phase o f expansion. The distance to M81 from the expansion parallax of SN 199 3J is D = zeta(3.5 +/- 0.3) Mpc, where zeta less than or equal to 1 is the scattering parameter. The distance to M81 from the expanding phot osphere method is D = zeta(3.48-0.09 delta t(0) + 0.09 delta R(0) +/- 0.20) Mpc, where delta t(0) is the difference between the actual time of shock breakout and JD = 2,449,074.6 in tenths of a day, and delta R (0) is the difference between the actual radius of the progenitor star and 3.86 x 10(13) cm, in units of 10(13) cm. In order to be consisten t with the Cepheid distance, these estimates indicate that SN 1993J ha d zeta similar to 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 R(0) greater than or similar to 4 .7 x 10(13) cm = 675 R..