Illustration of ejecta properties computed by CMFGEN for two different distributions of ⁵⁶Ni. Top: profile of the ⁵⁶Ni mass fraction versus velocity for the model with a ⁵⁶Ni-rich shell centered at 4000 km s⁻¹ and the original model without it. Both models have the same total ⁵⁶Ni mass of 0.08 M_⊙. Middle: electron-density profile for the models in the top panel and computed with CMFGEN at 200 d after explosion. The radial electron-scattering optical depth associated with the ⁵⁶Ni-rich shell is 0.15. Bottom: same as the middle panel, but now showing the mean number of electrons per nucleon γ_e, which is equal to N_eĀm_H∕ρ, where ρ is the mass density and Ā is the mean atomic weight of the material. Conditions are far from complete ionization (especially for hydrogen), so the ionization and the electron density could be enhanced even further (see Sect. 1 for a discussion).