2017ApJ...845..176B

We present a radiative transfer code to model the nebular phase spectra of supernovae (SNe) in non-LTE (NLTE). We apply it to a systematic study of SNe Ia using parameterized 1D models and show how nebular spectral features depend on key physical parameters, such as the time since explosion, total ejecta mass, kinetic energy, radial density profile, and the masses of ⁵⁶Ni, intermediate-mass elements, and stable iron-group elements. We also quantify the impact of uncertainties in atomic data inputs. We find the following. (1) The main features of SN Ia nebular spectra are relatively insensitive to most physical parameters. Degeneracy among parameters precludes a unique determination of the ejecta properties from spectral fitting. In particular, features can be equally well fit with generic Chandrasekhar mass (M_ch), sub-M_Ch, and super-M_Ch models. (2) A sizable (>=0.1 M_☉) central region of stable iron-group elements, often claimed as evidence for M_Ch models, is not essential to fit the optical spectra and may produce an unusual flat-top [CO III] profile. (3) The strength of [S III] emission near 9500 Å can provide a useful diagnostic of explosion nucleosynthesis. (4) Substantial amounts (>=0.1 M_☉) of unburned C/O mixed throughout the ejecta produce [O III] emission not seen in observations. (5) Shifts in the wavelength of line peaks can arise from line-blending effects. (6) The steepness of the ejecta density profile affects the line shapes, offering a constraint on explosion models. (7) Uncertainties in atomic data limit the ability to infer physical parameters.