The Search for Stable Nickel: Investigating the Origins of Type Ia Supernovae with Late-time Near-infrared Spectroscopy from the Carnegie Supernova Project-II

Abstract Producing stable 58 Ni in Type Ia supernovae (SNe Ia) requires sufficiently high-density conditions that are not predicted for all origin scenarios, so examining the distribution of 58 Ni using the near-infrared (NIR) [Ni II ] 1.939 μ m line may observationally distinguish between possible progenitors and explosion mechanisms. We present 79 telluric-corrected NIR spectra of 22 low-redshift SNe Ia from the Carnegie Supernova Project-II, ranging from +50 to +505 days, including 31 previously unpublished spectra. We introduce the Gaussian Peak Ratio, a detection parameter that confirms the presence of the NIR [Ni II ] 1.939 μ m line in eight SNe in our sample. Nondetections occur at earlier phases (≤+100 days) when the NIR Ni line has not emerged yet or in low signal-to-noise spectra, yielding inconclusive results. Subluminous 86G-like SNe Ia show the earliest NIR Ni features around ∼+50 days, whereas normal-bright SNe Ia do not exhibit NIR Ni until ∼+150 days. NIR Ni features detected in our sample have low peak velocities ( v ∼ 1200 km s −1 ) and narrow line widths (≤3500 km s −1 ), indicating stable 58 Ni is centrally located. This implies high-density burning conditions in the innermost regions of SNe Ia and could be due to higher mass progenitors (i.e., near-M ch ). NIR spectra of the nearly two dozen SNe Ia in our sample are compared to various model predictions and paired with early-time properties to identify ideal observation windows for future SNe Ia discovered by upcoming surveys with Rubin-LSST or the Roman Space Telescope.