THE UNUSUAL SUPERNOVA SN1993J IN THE GALAXY M81

SUPERNOVA 1993J in the galaxy M81 is the second-brightest type II supe rnova observed this century, surpassed only by SN1987A in the Large Ma gellanic Cloud. Here we report the evolution of the photometric and sp ectral properties of SN1993J for the first 50 days following its disco very. The behaviour of this supernova is unusual, showing features typ ical of type II supernovae near the initial maximum, but with the stro ng helium lines characteristic of type Ib supernovae at later times. T his implies that the progenitor star had an unusually thin hydrogen en velope (compared to normal type II progenitors), suggesting that signi ficant mass loss had taken place before the explosion. Application of an expanding photosphere model1 to our data provides an estimate of th e distance to the supernova of 2.6+/-0.4 Mpc, broadly consistent with the distance to M81 determined using Cepheid variable stars2. Supernov a models that more closely match the atypical spectral features of SN1 993J may change the inferred distance, and should provide better const raints on the structure of the progenitor.