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.
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