New 26P(p, γ)27S Thermonuclear Reaction Rate and Its Astrophysical Implications in the rp-process

Accurate nuclear reaction rates for P-26(p, gamma)S-27 are pivotal for a comprehensive understanding of the rp-process nucleosynthesis path in the region of proton-rich sulfur and phosphorus isotopes. However, large uncertainties still exist in the current rate of P-26(p, gamma)S-27 because of the lack of nuclear mass and energy level structure information for S-27. We reevaluate this reaction rate using the experimentally constrained S-27 mass, together with the shell model predicted level structure. It is found that the P-26(p, gamma)S-27 reaction rate is dominated by a direct capture reaction mechanism despite the presence of three resonances at E = 1.104, 1.597, and 1.777 MeV above the proton threshold in S-27. The new rate is overall smaller than the other previous rates from the Hauser-Feshbach statistical model by at least 1 order of magnitude in the temperature range of X-ray burst interest. In addition, we consistently update the photodisintegration rate using the new S-27 mass. The influence of new rates of forward and reverse reaction in the abundances of isotopes produced in the rp-process is explored by postprocessing nucleosynthesis calculations. The final abundance ratio of S-27/P-26 obtained using the new rates is only 10% of that from the old rate. The abundance flow calculations show that the reaction path P-26(p, gamma)S-27(beta (+),nu)P-27 is not as important as previously thought for producing P-27. The adoption of the new reaction rates for P-26(p, gamma)S-27 only reduces the final production of aluminum by 7.1% and has no discernible impact on the yield of other elements.

Automated summary

Accurate nuclear reaction rates for P-26(p, gamma)S-27 are crucial for understanding rp-process nucleosynthesis. In this study, the reaction rate is reevaluated using experimentally constrained S-27 mass and shell model predicted level structure. The new rate is found to be significantly smaller than previous rates and shows that the reaction is dominated by direct capture mechanism. The adoption of the new reaction rates has a limited impact on the abundances of isotopes produced in the rp-process.