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 56 Ni 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.