Stellar <i>β</i> ⁻-decay Rate of ⁶³Ni and Its Impact on the <i>s</i>-process Nucleosynthesis in Massive Stars

Abstract The β − -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 β − -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 K. Takahashi &amp; K. Yokoi 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 ∼ 90 in a star with an initial mass of 25 M ⊙ and solar metallicity. We find that the new rates can lead to 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 = 2 M ⊙ . The enhancement of the decay rate of 63 Ni increases the weak s -process efficiency of nuclei after 65 Cu.