[Co III] versus Na I D in Type Ia supernova spectra

The high metal content and fast expansion of supernova (SN) Ia ejecta lead to considerable line overlap in their optical spectra. Uncertainties in composition and ionization further complicate the process of line identification. In this paper, we focus on the 5900 Å emission feature seen in SN Ia spectra after bolometric maximum, a line which in the last two decades has been associated with [Co III] 5888 Å or Na I D. Using non-LTE time-dependent radiative-transfer calculations based on Chandrasekhar-mass delayed-detonation models, we find that Na I D line emission is extremely weak at all post-maximum epochs. Instead, we predict the presence of [Co III] 5888 Å after maximum in all our SN Ia models, which cover a range from 0.12 to 0.87 M⊙ of ⁵⁶Ni. We also find that the [Co III] 5888 Å forbidden line is present within days of bolometric maximum, and strengthens steadily for weeks thereafter. Both predictions are confirmed by observations. Rather than trivial taxonomy, these findings confirm that it is necessary to include forbidden-line transitions in radiative-transfer simulations of SNe Ia, both to obtain the correct ejecta cooling rate and to match observed optical spectra.