Analysis of the late optical spectra of SN 1993J

We have used a detailed non-LTE synthetic spectrum code to interpret Royal Greenwich Observatory archive spectra from the nebular phase of SN 1993J. In general, we obtain good agreement between the observed optical 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 with 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 material is mixed inward to similar to 1000 km s(-1). We find that the observed spectra are consistent with a symmetric explosion in which similar to 0.5 M. of oxygen occupies a spherical shell between 1000-4000 km s(-1). Observations interpreted earlier as evidence for large asymmetries are better explained by line blending effects. In particular, scattering of [O I] lambda lambda 6300, 6364 by H alpha in the hydrogen envelope strongly affects the [O I] profile. The width of the H alpha 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 hydrogen 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 envelope is significantly helium enriched. After similar to 350 days, an additional power source is necessary to reproduce the observed Her luminosity. It is likely that X-rays from the circumstellar interaction provide the necessary energy input.