Stable nickel production in type Ia supernovae: A smoking gun for the progenitor mass?
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Date
2022
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Abstract
Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit (M-Ch approximate to 1.4 M-circle dot) 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 (greater than or similar to 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 angstrom and 1.94 mu 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, Ni-58) 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-circle dot for the Ni-58 isotope), but we note that the Ni-58 yield in sub-M-Ch events systematically exceeds 0.01 M-circle dot for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction Co-57(p, gamma)Ni-58 is the dominant production mode for Ni-58 in both M-Ch and sub-M-Ch models, while the alpha-capture reaction on Fe-54 has a negligible impact on the final Ni-58 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 Ni-56 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.
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 angstrom and 1.94 mu 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, Ni-58) 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-circle dot for the Ni-58 isotope), but we note that the Ni-58 yield in sub-M-Ch events systematically exceeds 0.01 M-circle dot for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction Co-57(p, gamma)Ni-58 is the dominant production mode for Ni-58 in both M-Ch and sub-M-Ch models, while the alpha-capture reaction on Fe-54 has a negligible impact on the final Ni-58 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 Ni-56 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.
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Keywords
supernovae: general, nuclear reactions, nucleosynthesis, abundances, supernovae: individual: SN 2017bzc, radiative transfer