SN 1993J: The optical properties of its progenitor

We present a homogeneous set of UBVR_CI_C photometry of the progenitor of SN 1993J in M81. The photometry is inconsistent with the spectral energy distribution of a single star for any reasonable extinction (A_v<3 mag). Models of the spectral energy distribution which include 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 photometry. The progenitor image is embedded in weak extended emission, which further supports a composite model, and an OB association contamination model in particular. Stellar evolution theory, along with the plausibility of the OB association contamination model, favors the choice of the red star as the progenitor of SN 1993J. The best fitting models for the red star indicate that it is a K0I with M_bol≃-7.8. The allowed range (95% confidence) of the optical parameters for representative 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 canonical stellar evolution scenario in which M supergiants are the progenitors of Type II supernovae. Rather, some evolution to the blue, presumably following the M supergiant phase, is required in order to match our observations. Two factors which are likely to be important in explaining 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. Our estimate for the mass of the red star ( ≃ 17script M sign_⊙) 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≈ 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 light curve of SN 1993J have led several authors to conclude that the progenitor 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.