A CARBON-OXYGEN STAR AS PROGENITOR OF THE TYPE-IC SUPERNOVA 1994I

SUPERNOVAE are classified observationally as type I (which have no hyd rogen emission lines in their optical spectra) or type II (which do ha ve hydrogen lines), and are further subdivided(1) into types Ia, Ib, I c, IIP, IIL and IIb. Type II supernovae are generally thought to resul t from the explosion of massive stars, and type Ia supernovae from whi te-dwarf stars that have accreted sufficient mass from a companion to trigger explosion(1,2). The origins of types Ib and Ic are much less c lear. The Ic class has been particularly controversial, with two main alternatives: the explosion of a massive star that has lost its hydrog en and helium envelopes through fast stellar winds, exposing the carbo n- and oxygen-rich core; or the transfer of material to a companion, l eaving a small, bare carbon-oxygen (C + O) star which subsequently exp lodes(3,4). We show here that the observed characteristics of SN1994I (in the nearby galaxy NGC5194), which has recently(5,6) been classifie d as type Ic, are best understood on the basis of the second of the tw o alternatives above: that the progenitor was in a close binary system and lost its outer envelopes, leaving a C + O star (of mass approxima te to 2 solar masses) which exploded when its iron core collapsed.