Abstract
Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit (MCh ≈ 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 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 [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 MCh and sub-MCh 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 MCh delayed-detonation and sub-MCh 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 MCh explosions at solar metallicity (typically 0.02–0.08 M⊙ for the 58Ni isotope), but we note that the 58Ni yield in sub-MCh 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 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 [Ni 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 [Ni 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. [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 MCh and sub-MCh progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni II] lines would most likely result from a Chandrasekhar-mass progenitor.
Highlights
We studied the explosive nucleosynthesis of stable nickel and its dominant isotope 58Ni in SNe Ia to test its use as a diagnostic of the progenitor white dwarf (WD) mass
Among all reactions ending in 58Ni, we find that the radiative proton-capture reaction 57Co(p, γ)58Ni mostly determines the final 58Ni abundance in both MCh and subMCh models
The systematic absence of [Ni ii] lines in late-time spectra of sub-MCh models is due to the higher Ni ionization in the inner ejecta, where Ni iii dominates over Ni ii
Summary
One exception is the 1D MCh delayed-detonation model 5p0_Z0p014, published here for the first time, whose WD progenitor results from the evolution of a 5 M star at solar metallicity (Z = 0.014; Asplund et al 2009). The explosive phase was simulated using the same hydrodynamics and nucleosynthesis code as the sub-MCh detonation models 1p06_Z2p25e-2 and 0p88_Z2p25e-2 from a 1.06 M and 0.88 M WD progenitor, respectively, at slightly super-solar metallicity (Z = 0.025 ≈ 1.6 Z ; Bravo et al 2019). For a WD composed of only 12C, 16O, 22Ne, and 56Fe (i.e. X(12C) + X(16O) = 1 − X(22Ne) − X(56Fe)), the electron fraction is (see Kushnir et al 2020): Ye. time spectra for the low-luminosity MCh delayed-detonation model DDC25 and the sub-MCh detonation model SCH2p0 are from Blondin et al (2018).
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