Stellar ${\beta }^{-}$-decay rate of ${}^{63}$Ni and its impact on the $s$-process nucleosynthesis in massive stars

Abstract

The ${\beta }^{-}$-decay rate of ${}^{63}$Ni, an important branching point, affects the subsequent nucleosynthesis in the weak component of the slow-neutron capture process (weak $s$-process). To evaluate the impact of the uncertainties of stellar lifetime of ${}^{63}$Ni on abundances, we calculate the contribution to ${\beta }^{-}$-decay rates from its excited states using the large-scale shell model with various interactions and also explore the atomic effects in the highly ionized plasma. In the core He burning stage and the shell C burning stage of massive stars, our new rates can be larger than those from Takahashi and Yokoi(1987) by up to a factor of 4 and 6, respectively. We evaluate the impact of the stellar decay rates of ${}^{63}$Ni on the nucleosynthesis of $A=60\sim 90$ in a star with an initial mass of 25 $M_{⨀}$ and solar metalicity. We find that the new rates can lead to the abundance changes of ${}^{64}$Ni, ${}^{63}$Cu, ${}^{65}$Cu, ${}^{64}$Zn, ${}^{66}$Zn, ${}^{67}$Zn, and ${}^{68}$Zn by up to $18\%$, $14\%$, $7\%$, $98\%$, $16\%$, $15\%$, and $13\%$, respectively, after the shell C burning stage at the Lagrangian mass coordinate $M_r=2M_{⨀}$. The enhancement of the decay rate of ${}^{63}$Ni increases the weak $s$-process efficiency of nuclei after ${}^{65}$Cu.

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