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

We present near-infrared (NIR) spectra for Type Ia supernovae at epochs of 13to 338 days after maximum blue light. Some contemporary optical spectra arealso shown. All the NIR spectra exhibit considerable structure throughout theJ-, H- and K-bands. In particular they exhibit a flux `deficit' in the J-bandwhich persists as late as 175 days. This is responsible for the well-known redJ-H colour. To identify the emission features and test the $^{56}$Ni hypothesisfor the explosion and subsequent light curve, we compare the NIR and opticalnebular-phase data with a simple non-LTE nebular spectral model. We find thatmany of the spectral features are due to iron-group elements and that theJ-band deficit is due to a lack of emission lines from species which dominatethe rest of the IR/optical spectrum. Nevertheless, some emission is unaccountedfor, possibly due to inaccuracies in the cobalt atomic data. For somesupernovae, blueshifts of 1000--3000 km/s are seen in infrared and opticalfeatures at 3 months. We suggest this is due to clumping in the ejecta. Theevolution of the cobalt/iron mass ratio indicates that $^{56}$Co-decaydominates the abundances of these elements. The absolute masses of iron-groupelements which we derive support the basic thermonuclear explosion scenario forType Ia supernovae. A core-collapse origin is less consistent with our data.Comment: 33 Latex pages, 12 Postscript figures: accepted by Monthly Notices of the Royal Astronomical Societ