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 (MCh1.4 Ma) 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 MCh and sub-MCh events. In particular, it is thought that the higher-density ejecta of MCh WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Nia-II] lines in late-time spectra (150 d past explosion). Aims. We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from MCh and sub-MCh progenitors. We explore the potential for lines of [Nia-II] in the optical an near-infrared (at 7378 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 MCh delayed-detonation and sub-MCh detonation models, we studied the synthesis of stable Ni isotopes (in particular, 58Ni) and investigated the formation of [Nia-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 MCh explosions at solar metallicity (typically 0.02a0.08 Ma for the 58Ni isotope), but we note that the 58Ni yield in sub-MCh events systematically exceeds 0.01 Ma for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction 57Co(p,aγ)58Ni is the dominant production mode for 58Ni in both MCh and sub-MCh 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 [Nia-II] lines in late-time spectra of sub-MCh 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 [Nia-II] lines predicted in our 1D MCh models are completely suppressed when 56Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements. Conclusions. [Nia-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 MCh and sub-MCh progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Nia-II] lines would most likely result from a Chandrasekhar-mass progenitor.