SN 1993J - THE OPTICAL-PROPERTIES OF ITS PROGENITOR

We present a homogeneous set of UBVR(C)I(C) photometry of the progenit or of SN 1993J in M81. The photometry is inconsistent with the spectra l energy distribution of a single star for any reasonable extinction ( A(V) < 3 mag). Models of the spectral energy distribution which includ e the spectra of a red and a blue star, or of a red star plus the net spectrum of an OB association, give excellent agreement with our photo metry. The progenitor image is embedded in weak extended emission, whi ch further supports a composite model, and an OB association contamina tion model in particular. Stellar evolution theory, along with the pla usibility of the OB association contamination model, favors the choice of the red star as the progenitor of SN 1993J. The best fitting model s for the red star indicate that it is a K0I with Mbol similar to-7.8. The allowed range (95% confidence) of the optical parameters for repr esentative extinction values is: spectral type G8I-K5I; M(bol)=-6.8-7. 9, for A(V)=0.5 mag, and spectral type G2I-K5I; M(bol)=-7.4-8.2, for A (V)=1.0 mag. The properties of the red star do not support the canonic al stellar evolution scenario in which M supergiants are the progenito rs of Type II supernovae. Rather, some evolution to the blue, presumab ly following the M supergiant phase, is required in order to match our observations. Two factors which are likely to be important in explain ing the red star's unexpected location in the H-R diagram are that it may be in a mass-transfer binary and that it is probably metal rich. O ur estimate for the mass of the red star ( similar to M(.)) indicates that it had an initial mass very similar to (the much bluer) SN 1987A. We note that the LMC and inner region M81 exhibit an approximate to 0 .7 dex difference in metallicity. The assumption that this metallicity difference applies to SN 1987A and SN 1993J suggests that metallicity may play an important role in determining the extent of blueward post -RSG evolution. The secondary maximum and lack of a plateau in the lig ht curve of SN 1993J have led several authors to conclude that the pro genitor of SN 1993J had a very low mass hydrogen envelope. It has been suggested that removal of the hydrogen envelope by mass transfer in a binary system could result in post-RSG evolution toward the blue.