SN 1993J: A type IIb supernova

The evolution of the bright Type II supernova discovered last year in M81, SN 1993J, is consistent with that expected for the explosion of a star which on the main sequence had a mass of 13-16 M_⊙ but which, owing to mass exchange with a binary companion (a initially ∼3-5 AU, depending upon the actual presupernova radius and the masses of the two stars) lost almost all of its hydrogen-rich envelope during late helium burning. At the time of explosion, the helium core mass was 4.0 ± 0.5 M_⊙ and the hydrogen envelope, 0.20 ± 0.05 M⊙. The envelope was helium and nitrogen-rich (carbon-deficient) and the radius of the star, 4 ± 1 × 10¹³ cm. The luminosity of the presupernova star was 3 ± 1 × 10³⁸ ergs s^-1, with the companion star contributing an additional ∼ 10³⁸ ergs s^-1. The star may have been a pulsating variable at the time of the explosion. For an explosion energy near 10⁵¹ ergs (KE at infinity) and an assumed distance of 3.3 Mpc, a mass of ⁵⁶Ni in the range 0.07 ± 0.01 M_⊙ was produced and ejected. This prescription gives a light curve which compares favorably with the bolometric observations. Color photometry is more restrictive and requires a model in which the hydrogen-envelope mass is low and the mixing of hydrogen inward has been small, but in which appreciable ⁵⁶Ni has been mixed outward into the helium and heavy-element core. It is possible to obtain good agreement with B and V light curves during the first 50 days, but later photometry, especially in bands other than B and V, will require a non-LTE spectral calculation for comparison. Based upon our model, we predict a flux of ∼10^-5 (3.3 Mpc/D)² photons cm^-2 s^-1 in the 847 kcV line of ⁵⁶Co at peak during 1993 August. It may be easier to detect the Comptonized continuum which peaks at a few times 10^-4 photons s^-1 cm^-2 MeV^-1 at 40 keV a few months after the explosion (though neither of these signals were, or should have been, detected by the Compton Gamma-Ray Observatory). The presupernova star was filling its Roche lobe at the time of the explosion and thus its envelope was highly deformed (about 3:2). The companion star is presently embedded in the supernova, but should become visible at age 3 yr (perhaps earlier in the ultraviolet) when the supernova has faded below 10³⁸ ergs s^-1. Indeed, if "kicks" have not played an important role, it is still bound to the neutron star.