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 (cid:12) ) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable iron-group elements synthe- sized 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 ( (cid:38) 150d 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 7378Å and 1.94 µ m) 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, 58 Ni) 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 e ffi cient in M Ch explosions at solar metallicity (typically 0.02– 0.08 M (cid:12) for the 58 Ni isotope), but we note that the 58 Ni yield in sub- M Ch events systematically exceeds 0.01 M (cid:12) for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction 57 Co( p ,γ ) 58 Ni is the dominant production mode for 58 Ni in both M Ch and sub- M Ch models, while the α -capture reaction on 54 Fe has a negligible impact on the final 58 Ni 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 56 Ni is su ffi ciently mixed with the innermost layers, which are rich in stable iron-group elements. stable among Ch which is implicitly 100% when not given. The decay mode (cid:15) refers to electron capture (EC). Although its half-life is less than 1s, we report the 64 Co → 64 Ni decay since it is part of the 64 Fe → 64 Co → 64 Ni decay chain. We do not consider excited nuclear isomer states of 60m Mn, 60m Co, or 62m Co.