The progenitor of supernova 1993J revisited

From Hubble Space Telescope images with 0.05 resolution, we identify four stars brighter than V = 25 mag within 2.5 of SN 1993J in M81, which contaminated previous ground-based brightness estimates for the supernova progenitor. Correcting for the contamination, we find that the energy distribution of the progenitor is consistent with that of an early K-type supergiant star with M-V approximate to -7.0 +/- 0.4 mag and an initial mass of 13-22 M.. The brightnesses of the nearby stars are sufficient to account for the excess blue light seen from the ground in preexplosion observations. Therefore, the SN 1993J progenitor did not necessarily have a blue companion, although by 2001, fainter blue stars are seen in close proximity to the supernova. These observations do not strongly limit the mass of a hypothetical companion. A blue dwarf star with a mass up to 30 M. could have been orbiting the progenitor without being detected in the ground-based images. Explosion models and observations show that SN 1993J progenitor had a helium-rich envelope. To test whether the helium abundance could influence the energy distribution of the progenitor, we calculated model supergiant atmospheres with a range of plausible helium abundances. The models show that the presupernova colors are not strongly affected by the helium abundance longward of 4000 Angstrom, and abundances ranging between solar and 90% helium (by number) are all consistent with the observations.