The Impact of the New $^{65\!}$As(p,$\gamma$)$^{66\!}$Se Reaction Rate on the Two-Proton Sequential Capture of $^{64}\!$Ge, Weak GeAs Cycles, and Type-I X-Ray Bursts such as the Clocked Burster GS 1826$-$24

Abstract

We re-assess $^{65}$As(p,$\gamma$)$^{66}$Se reaction rates based on a set of proton thresholds of $^{66}$Se, $S_\mathrm{p}$($^{66}$Se), estimated from the experimental mirror nuclear masses, theoretical mirror displacement energies, and full $pf$-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, that closely reproduces the observed GS 1826$-$24 clocked bursts, the present forward and reverse $^{65}$As(p,$\gamma$)$^{66}$Se reaction rates based on a selected $S_\mathrm{p}$($^{66}$Se) = 2.469$\pm$0.054 MeV, and the latest $^{22}$Mg($\alpha$,p)$^{25}$Al, $^{56}$Ni(p,$\gamma$)$^{57}$Cu(p,$\gamma$)$^{58}$Zn, $^{55}$Ni(p,$\gamma$)$^{56}$Cu, and $^{64}$Ge(p,$\gamma$)$^{65}$As reaction rates, we find that though the GeAs cycles is weakly established in the rapid-proton capture process path, the $^{65}$As(p,$\gamma$)$^{66}$Se reaction still strongly characterizes the burst tail end due to the two-proton sequential capture on $^{64}$Ge, not found by Cyburt et al. (2016) sensitivity study. The $^{65}$As(p,$\gamma$)$^{66}$Se reaction influences the abundances of nuclei $A$ = 64, 68, 72, 76, and 80 up to a factor of 1.4. The new $S_\mathrm{p}$($^{66}$Se) and the inclusion of the updated $^{22}$Mg($\alpha$,p)$^{25}$Al reaction rate increases the production of $^{12}$C up to a factor of $4.5$ that is not observable and could be the main fuel for superburst. The waiting point status of and two-proton sequential capture on $^{64}$Ge, weak-cycle feature of GeAs at region heavier than $^{64}$Ge, and impact of other possible $S_\mathrm{p}$($^{66}$Se) are also discussed.