Carnegie Supernova Project-II: Using Near-infrared Spectroscopy to Determine the Location of the Outer ⁵⁶Ni 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, v _edge, of the prominent Fe/Co/Ni-peak H-band emission feature, which is quantitatively measured. The v _edge 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 v _edge is correlated with the color-stretch parameter, s _BV, 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 v _edge 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.