Non-thermal excitation and ionization in supernovae

TL;DR

This paper integrates non-thermal excitation and ionization processes into supernova radiative transfer modeling, demonstrating their importance for accurate spectral predictions, especially in nebular phases, and comparing results with SN 1987A observations.

Contribution

The authors incorporate non-thermal processes into the CMFGEN code, enabling comprehensive supernova spectral modeling across all types and epochs.

Findings

Non-thermal processes significantly enhance certain spectral lines.

Inclusion of FeI affects optical spectra through line-blanketing.

Model spectra show broad agreement with SN 1987A observations.

Abstract

We incorporate non-thermal excitation and ionization processes arising from non-thermal electrons that result from \gamma-ray energy deposition, into our radiative transfer code CMFGEN. The non-thermal electron distribution is obtained by solving the Spencer-Fano equation using the procedure of Kozma & Fransson (1992). We applied the non-thermal calculations to the blue supergiant explosion model whose early evolution was studied in Dessart & Hillier (2010). Non-thermal processes generally increase excitation and ionization and decrease the temperature of the ejecta. We confirm that non-thermal processes are crucial for modeling the nebular spectra. Both optical HI and HeI lines are significantly strengthened. While optical HeI lines are not easily discerned in observational spectra due to severe blending with other lines, HeI 2.058 \mu m provides an excellent opportunity to infer the influence of non-thermal processes. We also discuss the processes controlling the formation of the HeI lines during the nebular epoch. Most lines of other species are only slightly affected. We also show that the inclusion of FeI has substantial line-blanketing effects on the optical spectra. Our model spectra and synthetic light curves are compared to the observations of SN 1987A. The spectral evolution shows broad agreement with the observations, especially H\alpha. The uncertainties of the non-thermal solver are studied, and are expected to be small. With this new addition of non-thermal effects in CMFGEN, we now treat all known important processes controlling the radiative transfer of a supernova ejecta, whatever the type and the epoch.