Ionization of type Ia supernovae: Electron impact, photon impact, or charge transfer?

The ionization structure of the nebular Type Ia supernova iron core is determined by treating important atomic processes with the best available evaluated atomic data and by assuming both Chandrasekhar-mass and sub-Chandrasekhar-mass white dwarf explosion models. The electron impact ionization rates are calculated by analyzing the energetic electron energy deposition processes. Charge transfer processes between neutral and ionized iron-group elements are included in the calculations using the recently available rate coefficients. The effect of photoionization due to the recombination photons is critically evaluated by carrying out a detailed cascade calculation using the recently improved state-specific rate coefficients of radiative recombination and cross sections of photoionization of iron. It is concluded that the ionization structure is mainly controlled by a combination of the energetic electron impact ionization and charge transfer, while the photoionization plays only a minor role and its effect on the synthetic spectra is small.