Infrared and optical spectroscopy of type Ia supernovae in the nebular phase

We present near-infrared (NIR) spectra for Type Ia supernovae al epochs of 13 to 338 d after maximum blue light. Some contemporary optical spectra are also shown. All the NIR spectra exhibit considerable structure throughout the J, H and K bands. In particular, they exhibit a flux 'deficit' in the J band which persists as late as 175 d. This is responsible for the well-known red J-H colour, To identify the emission features and test the Ni-56 hypothesis for the explosion and subsequent light curve, we compare the NIR and optical nebular-phase data with a simple non-LTE nebular spectral model. We find that many of the spectral features are due to iron-group elements, and that the J-band deficit is due to a lack of emission lines from species that dominate the rest of the IR/optical spectrum. Nevertheless, some emission is unaccounted for, possibly due to inaccuracies in the cobalt atomic data. For some supernovae, blueshifts of 1000-3000 km s(-1) are seen in infrared and optical features at 3 months. We suggest that this is due to clumping in the ejecta. The evolution of the cobalt/iron mass ratio indicates that Co-56-decay dominates the abundances of these elements, The absolute masses of iron-group elements which we derive support the basic thermonuclear explosion scenario for Type Ia supernovae. A core-collapse origin is less consistent with our data.