Evaluation of electron capture reaction rates in Ni isotopes in stellar environments

Abstract

Electron capture rates in Ni isotopes are studied in stellar environments, that is, at high densities and high temperatures during the core-collapse and postbounce explosive nucleosynthesis in supernovae. Reaction rates in $^{58}\mathrm{Ni}$ and $^{60}\mathrm{Ni}$, as well as in $^{56}\mathrm{Ni}$, $^{62}\mathrm{Ni}$, and $^{64}\mathrm{Ni}$, are evaluated by shell-model calculations with the use of a new shell-model Hamiltonian in the $\mathit{fp}$ shell, GXPF1J. While the previous shell-model calculations failed to reproduce the measured peaks of Gamow-Teller strength in $^{58}\mathrm{Ni}$ and $^{60}\mathrm{Ni}$, the present new Hamiltonian is found to reproduce them very well, as well as the capture rates obtained from the observed strengths. Strengths and energies of the Gamow-Teller transitions in $^{56}\mathrm{Ni}$, $^{62}\mathrm{Ni}$, and $^{64}\mathrm{Ni}$ are also found to be consistent with the observations.