GAMMA-RAY DEPOSITION AND NONTHERMAL EXCITATION IN SUPERNOVAE

We calculate the gamma-ray deposition in supernovae by solving the Spencer-Fano equation. Ionization, excitation, and heating rates are presented for the different chemical composition zones of a core collapse supernova, as well as for a solar composition applicable to for example, active galactic nuclei. We also discuss the thermalization in pure helium, oxygen, and iron plasmas. The latter is of particular interest for Type Ia supernovae. Convenient analytical expressions are given to facilitate the use of these results for the calculation of the physical conditions and emission from supernovae. We also discuss the spectral characteristics of the emission following nonthermal excitation and ionization in the supernova. In particular, we calculate the fraction of the absorbed gamma-ray energy which is reemitted as UV photons with energy above 3.4 eV, the n = 2 threshold of hydrogen. We apply these results to the ionization of hydrogen and the formation of the H-alpha line. Good qualitative and quantitative agreement between the model and observations of SN 1987A are found. In particular, we find that absorption of UV photons in the Balmer continuum dominates the excitation before approximately 500 days, while direct nonthermal excitation from the ground state dominates thereafter. We also discuss how the wing of the H-alpha line can be used as a probe of the density in the envelope.