How do Type Ia Supernova Nebular Spectra Depend on Explosion Properties? Insights from Systematic non-LTE Modeling

Abstract: We present a radiative transfer code to model the nebular phase spectra of supernovae (SNe) in non-local thermodynamic equilibrium (NLTE). We apply it to a systematic study of Type Ia SNe 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 (IMEs), and stable iron-group elements (IGEs). We also quantify the impact of uncertainties in atomic data inputs. Among the results of the study are: (1) The main features of SNe 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 Msun) central region of stable IGEs, often claimed as evidence for M_Ch models, is not essential to fit the optical spectra and may produce an unusual flat-top [CoIII] profile; (3) The strength of [SIII] emission near 9500A can provide a useful diagnostic of explosion nucleosynthesis; (4) Substantial amounts (~0.1 Msun) of unburned C/O mixed throughout the ejecta produce [OIII] emission not seen in observations; (5) Shifts in the wavelength of line peaks, sometimes used to infer ejecta geometry, can also arise from line blending effects; (6) The steepness of the ejecta density profile affects the line shapes, with flatter slopes providing better fits to the observations of SN 2011fe, offering a constraint on explosion models; (7) Uncertainties in atomic data affect spectral line ratios by ~30%, a level similar to the effect of varying physical parameters.