Understanding nebular spectra of Type Ia supernovae

In this study, we present one-dimensional, non-local-thermodynamic-equilibrium, radiative transfer simulations (using CMFGEN) in which we introduce micro-clumping at nebular times into two Type Ia supernova ejecta models. We use one sub-Chandrasekhar (sub-M-Ch) ejecta model with 1.04 M-circle dot and one Chandrasekhar (M-Ch) ejecta model with 1.40 M-circle dot. We introduce clumping factors f = 0.33, 0.25, and 0.10, which are constant throughout the ejecta, and compare results to the unclumped f = 1.0 case. We find that clumping is a natural mechanism to reduce the ionization of the ejecta, reducing emission from [Fe III], [Ar III], and [S III] by a factor of a few. For decreasing values of the clumping factor f, the [Ca II] lambda lambda 7291,7324 doublet became a dominant cooling line for our M-Ch model but remained weak in our sub-M-Ch model. Strong [Ca II] lambda lambda 7291,7324 indicates non-thermal heating in that region and may constrain explosion modelling. Due to the low abundance of stable nickel, our sub-M-Ch model never showed the [Ni II] 1.939-mu m diagnostic feature for all clumping values.