MODELING AND INTERPRETATION OF THE OPTICAL AND HST UV SPECTRUM OF SN 1993J

We present spherically symmetric, non-LTE, expanding model atmosphere synthetic spectra of SN 1993J and compare them to the ultraviolet and optical spectra obtained simultaneously on 1993 April 15 by the HST and at Lick Observatory. We are able to fit the optical and HST near-UV spectra with a variety of compositions, but find that our best fits are obtained with an enhanced helium abundance (Y = 0.8) and significant nonthermal ionization due to gamma rays produced by the decay of Ni-56. We find the effects of X-rays produced by thermal bremsstrahlung in the circumstellar material to have little effect on the spectrum. Our current models are unable to fit the HS-T far-UV region, predicting too little flux and strong, unseen Fe II features.