Early Observations of SN 1993J in M81 at McDonald Observatory

Abstract

The visual light and color curves of SN 1993J show behavior unlike that of other Type II supernovae. The initial flash peaked at V = 10.7 on JD = 2,449,076.8, 3 days following the shock outbreak which is estimated to have occurred at JD 2,449,074.5 = March 28.0 UT. High-dispersion spectra show 10 velocity components in each of the Na D lines. Early optical and IR spectra show a nearly blackbody continuum cooling from 14,900 +/- 1000 K to 11,800 +/- 700 K from March 31.2 to April 1.2 UT. Superposed on this continuum are broad, weak Balmer, Paschen, and He I lines. From blackbody fits to early spectra the color excess is estimated to be about E(B - V) = 0.15 +/- 0.02. The V light curve is reproduced by a model based on a 3.3 M_sun_ helium core of which the outer 0.4 M_sun_ is 20% hydrogen by mass. The recession of the photosphere through the thin H layer is completed at the first minimum. The second peak at V = 10.86 on JD 2,449,095.5 = April 18.0 is powered by radioactive heating. The subsequent decreasing rate of decline of the light curve may indicate the presence of a constant-luminosity source of about 10^41^ ergs s^-1^. Application of the expanding photosphere method to the early spectra gives a distance estimate of 4.2 +/- 0.6 Mpc, or DM = 28.1 +/- 0.4, but the result will depend on the detailed structure of the atmosphere.