[CoIII] versus NaID 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 5900A emission feature seen in
SN Ia spectra after bolometric maximum, a line which in the last two decades
has been associated with [CoIII]5888A or NaID. Using non-LTE time-dependent
radiative-transfer calculations based on Chandrasekhar-mass delayed-detonation
models, we find that NaID line emission is extremely weak at all post-maximum
epochs. Instead, we predict the presence of [CoIII]5888A after maximum in all
our SN Ia models, which cover a range from 0.12 to 0.87Msun of 56Ni. We also
find that the [CoIII]5888A 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.
