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 $A=60\sim90$ in a star with an initial mass of 25 $M_{ \bigodot}$ 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 $M_{ r}=2M_{ \bigodot}$. The enhancement of the decay rate of $^{ 63}$Ni increases the weak $s$-process efficiency of nuclei after $^{ 65}$Cu.
Categories: astro-ph.HE