SUPERNOVA-1993J AS A SPECTROSCOPIC LINK BETWEEN TYPE-II AND TYPE-IB SUPERNOVAE

SUPERNOVA 1993J in the nearby galaxy M81 is one of the closest-and hen ce brightest-supernovae to be witnessed this century. The early spectr um of SN1993J showed1-3 the characteristic hydrogen signature of type II supernovae, but its subsequent evolution is atypical for this class of supernova. Here we present optical and infrared spectra of SN1993J up to 43 days after outburst, which reveal the onset of the helium ab sorption and emission features more commonly associated with hydrogen- free type Ib supernovae. Corresponding model spectra show that the pro genitor star must have possessed an unusually thin (for type II supern ovae) hydrogen-rich envelope overlying a helium-rich mantle. Moreover, the supernova ejecta must have remained compositionally stratified, w ith little transport of the hydrogen-rich material down into the under lying helium layer, or mixing of heavier elements (such as radioactive Ni-56) outwards. SN1993J therefore represents a transition object bet ween hydrogen-dominated type II supernovae, and hydrogen-free, helium- dominated type Ib supernovae.