Impact of the New ⁶⁵As(p,γ)⁶⁶Se Reaction Rate on the Two-proton Sequential Capture of ⁶⁴Ge, Weak GeAs Cycles, and Type I X-Ray Bursts Such as the Clocked Burster GS 1826−24

Abstract We reassess the 65As(p,γ)66Se reaction rates based on a set of proton thresholds of 66Se, S p(66Se), estimated from the experimental mirror nuclear masses, theoretical mirror displacement energies, and full p f-model space shell-model calculation. The self-consistent relativistic Hartree–Bogoliubov theory is employed to obtain the mirror displacement energies with much reduced uncertainty, and thus reducing the proton-threshold uncertainty up to 161 keV compared to the AME2020 evaluation. Using the simulation instantiated by the one-dimensional multi-zone hydrodynamic code, Kepler, which closely reproduces the observed GS 1826−24 clocked bursts, the present forward and reverse 65As(p,γ)66Se reaction rates based on a selected S p(66Se) = 2.469 ± 0.054 MeV, and the latest 22Mg(α,p)25Al, 56Ni(p,γ)57Cu, 57Cu(p,γ)58Zn, 55Ni(p,γ)56Cu, and 64Ge(p,γ)65As reaction rates, we find that though the GeAs cycles are weakly established in the rapid-proton capture process path, the 65As(p,γ)66Se reaction still strongly characterizes the burst tail end due to the two-proton sequential capture on 64Ge, not found by the Cyburt et al. sensitivity study. The 65As(p,γ)66Se reaction influences the abundances of nuclei A = 64, 68, 72, 76, and 80 up to a factor of 1.4. The new S p(66Se) and the inclusion of the updated 22Mg(α,p)25Al reaction rate increases the production of 12C up to a factor of 4.5, which is not observable and could be the main fuel for a superburst. The enhancement of the 12C mass fraction alleviates the discrepancy in explaining the origin of the superburst. The waiting point status of and two-proton sequential capture on 64Ge, the weak-cycle feature of GeAs at a region heavier than 64Ge, and the impact of other possible S p(66Se) are also discussed.