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Stable Nickel Production in Type Ia Supernova Models

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Contact: F.X.Timmes
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Stable nickel production in type Ia supernovae: A smoking gun for the progenitor mass? (2022)

At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit ($M_\mathrm{Ch} \approx 1.4 M_{\odot}$) 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 $M_\mathrm{Ch}$ and sub-$M_\mathrm{Ch}$ events. In particular, it is thought that the higher-density ejecta of $M_\mathrm{Ch}$ WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni II] lines in late-time spectra ($\gtrsim$ 150 d past explosion).

In this article we study the explosive nucleosynthesis of stable nickel in SNs Ia resulting from $M_\mathrm{Ch}$ and sub-$M_\mathrm{Ch}$ progenitors. We explore the potential for lines of [Ni II] in the optical an near-infrared (at 7378 Å and 1.94 $\mu$m) in late-time spectra to serve as a diagnostic of the exploding WD mass. We confirm that stable Ni production is generally more efficient in $M_\mathrm{Ch}$ explosions at solar metallicity (typically 0.02--0.08 $M_{\odot}$ for the $^{58}\mathrm{Ni}$ isotope), but we note that the $^{58}\mathrm{Ni}$ yield in sub-$M_\mathrm{Ch}$ events systematically exceeds 0.01 $M_{\odot}$ for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction $^{57}\mathrm{Co}(\mathrm{p},\gamma)^{58}\mathrm{Ni}$ is the dominant production mode for $^{58}\mathrm{Ni}$ in both $M_\mathrm{Ch}$ and sub-$M_\mathrm{Ch}$ models, while the $\alpha$-capture reaction on $^{54}\mathrm{Fe}$ has a negligible impact on the final $^{58}\mathrm{Ni}$ yield. More importantly, we demonstrate that the lack of [Ni II] lines in late-time spectra of sub-$M_\mathrm{Ch}$ 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 $M_\mathrm{Ch}$ models are completely suppressed when $^{56}\mathrm{Ni}$ is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements. [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 $M_\mathrm{Ch}$ and sub-$M_\mathrm{Ch}$ progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni II] lines would most likely result from a Chandrasekhar-mass progenitor.


image
Density at peak temperature vs peak temperature in the $M_\mathrm{Ch}$ delayed-detonation model (filled circles) and the sub-$M_\mathrm{Ch}$ detonation models $M_\mathrm{WD}=1.06$ $M_{\odot}$ (squares) and $M_\mathrm{WD}=0.88$ $M_{\odot}$ (triangles). The wide blue band denotes the transition between incomplete Si burning and complete burning to NSE. The NSE region is further subdivided into normal and alpha-rich freeze-out regimes. The width of the bands correspond to variations in the post-burn cooling time scale.
image
Electron fraction Ye profile ≃ 30 min past explosion in the inner ejecta of the $M_\mathrm{Ch}$ delayed-detonation model (dashed) and the $M_\mathrm{WD}=1.06 M_{\odot}$ sub-$M_\mathrm{Ch}$ detonation model (solid). The markers correspond to the Ye of the nucleus (e.g. Ye = 28/58 ≃ 0.483 for $^{58}$Ni) and the interpolated velocity on the Ye profile for the $M_\mathrm{Ch}$ delayed-detonation model (both $^{56}\mathrm{Ni}$ and $^{64}$Ni are synthesized in this model but the Ye profile does not intersect the Ye value of either isotope).
image


See article's caption.