Carnegie Supernova Project-II: Using Near-infrared Spectroscopy to Determine the Location of the Outer 56 Ni in Type Ia Supernovae

We present the H-band wavelength region of 37 postmaximum light near-infrared spectra of three normal, nine transitional, and four subluminous type Ia supernovae (SNe Ia), extending from +5 days to +20 days relative to the epoch of B-band maximum. We introduce a new observable, the blue-edge velocity, vedge, of the prominent Fe/Co/Ni-peak H-band emission feature, which is quantitatively measured. The vedge parameter is found to decrease over subtype ranging from around −14,000 km s−1 for normal SNe Ia, to −10,000 km s−1 for transitional SNe Ia, down to −5000 km s−1 for the subluminous SNe Ia. Furthermore, inspection of the +10 ± 3 days spectra indicates that vedge is correlated with the color-stretch parameter, sBV, and hence with peak luminosity. These results follow the previous findings that brighter SNe Ia tend to have 56Ni located at higher velocities as compared to subluminous objects. As vedge is a model-independent parameter, we propose it can be used in combination with traditional observational diagnostics to provide a new avenue to robustly distinguish between leading SNe Ia explosion models.