Stable nickel production in type Ia supernovae: A smoking gun for the progenitor mass?

Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit (M_Ch ≈ 1.4 M_⊙) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable iron-group elements synthesized in the explosion has been suggested as a possible discriminant between M_Ch and sub-M_Ch events. In particular, it is thought that the higher-density ejecta of M_Ch WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni II] lines in late-time spectra (≳150 d past explosion).
Aims: We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from M_Ch and sub-M_Ch progenitors. We explore the potential for lines of [Ni II] in the optical an near-infrared (at 7378 Å and 1.94 μm) in late-time spectra to serve as a diagnostic of the exploding WD mass.
Methods: We reviewed stable Ni yields across a large variety of published SN Ia models. Using 1D M_Ch delayed-detonation and sub-M_Ch detonation models, we studied the synthesis of stable Ni isotopes (in particular, ⁵⁸Ni) and investigated the formation of [Ni II] lines using non-local thermodynamic equilibrium radiative-transfer simulations with the CMFGEN code.
Results: We confirm that stable Ni production is generally more efficient in M_Ch explosions at solar metallicity (typically 0.02-0.08 M_⊙ for the ⁵⁸Ni isotope), but we note that the ⁵⁸Ni yield in sub-M_Ch events systematically exceeds 0.01 M_⊙ for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction ⁵⁷Co(p, γ)⁵⁸Ni is the dominant production mode for ⁵⁸Ni in both M_Ch and sub-M_Ch models, while the α-capture reaction on ⁵⁴Fe has a negligible impact on the final ⁵⁸Ni yield. More importantly, we demonstrate that the lack of [Ni II] lines in late-time spectra of sub-M_Ch events is not always due to an under-abundance of stable Ni; rather, it results from the higher ionization of Ni in the inner ejecta. Conversely, the strong [Ni II] lines predicted in our 1D M_Ch models are completely suppressed when ⁵⁶Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements.
Conclusions: [Ni II] lines in late-time SN Ia spectra have a complex dependency on the abundance of stable Ni, which limits their use in distinguishing among M_Ch and sub-M_Ch progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni II] lines would most likely result from a Chandrasekhar-mass progenitor.