A radioactive excitation source model for the late time spectra of Type I supernovae

The synthetic spectrum of Fe+ and Fe++ is calculated as a function of electron temperature, assuming a Boltzmann population of the metastable levels of Fe+ and Fe++. This synthetic spectrum is compared with the spectrum of the Type I supernova 1972e in NGC 5253, taken 245 days after maximum. It is shown that good quantitative agreement can be obtained between the observed spectra and the synthetic spectra at an electron temperature of approx.4000 K with a ratio of Fe++ to Fe+ of approx.4 to 1. The radioactive excitation source model of late time spectra is used to interpret this result. It is shown that the ionic concentrations result from the ionization by primary electrons with energies of approx.1 MeV. The secondary electrons produced in the ionization process deposit their energy in the electron gas. The temperature of the electron gas is determined by the rate of energy deposited in the electron gas and rate of radiation of forbidden line radiation by Fe II and Fe III. Approximately 60% of the energy deposited should be in forbidden line radiation. The remaining energy should be mostly recombination radiation. The model predicts that a mass of approx.0.5 M/sub sun/ is required to produce the 4 to 1 ratio of Fe++ to Fe+. About the same mass of explosively synthesized Ni is required to produce the excitation source. These results are consistent with an interpretation that the supernova envelope at late time consists mostly of Fe. The evidence for an exterior He shell is discussed