ANALYSIS OF THE LATE OPTICAL-SPECTRA OF SN 1993J

We have used a detailed non-LTE synthetic spectrum code to interpret R oyal Greenwich Observatory archive spectra from the nebular phase of S N 1993J. In general, we obtain good agreement between the observed opt ical spectra and synthetic spectra based on the helium star explosion model 4H 47 of Nomoto et al., corresponding to a 3.2 M. helium core wi th 0.08 M. of Ni-56. The fit to the observed spectrum is improved when some of the iron-rich material is macroscopically mixed outward to a velocity of at least 3000 km s(-1), and when some of the oxygen-rich m aterial is mixed inward to similar to 1000 km s(-1). We find that the observed spectra are consistent with a symmetric explosion in which si milar to 0.5 M. of oxygen occupies a spherical shell between 1000-4000 km s(-1). Observations interpreted earlier as evidence for large asym metries are better explained by line blending effects. In particular, scattering of [O I] lambda lambda 6300, 6364 by H alpha in the hydroge n envelope strongly affects the [O I] profile. The width of the H alph a absorption feature constrains the velocity thickness of the hydrogen envelope, while the observed time evolution of this feature indicates that Ha became optically thin in 1993 October, constraining the hydro gen density. From these constraints and the observed Ha luminosity and line width, we conclude that the H/He envelope mass is 0.2-0.4 M. and that the bulk of this mass lies between 8500-10,000 km s(-1). The env elope is significantly helium enriched. After similar to 350 days, an additional power source is necessary to reproduce the observed Her lum inosity. It is likely that X-rays from the circumstellar interaction p rovide the necessary energy input.