THE RGB AND AGB STAR NUCLEOSYNTHESIS IN LIGHT OF THE RECENT 17O(p, α)14N AND 18O(p, α)15N REACTION-RATE DETERMINATIONS

In recent years, the Trojan Horse Method (THM) has been used to investigate the low-energy cross sections of proton-induced reactions on A = 17 and A = 18 oxygen isotopes, overcoming extrapolation procedures and enhancement effects due to electron screening. In particular, the strengths of the 20 keV and 65 keV resonances in the O-18(p, alpha)N-15 and O-17(p, alpha)N-14 reactions, respectively, have been extracted, as well as the contribution of the tail of the broad 656 keV resonance in the O-18(p, alpha)N-15 reaction inside the Gamow window. The strength of the 65 keV resonance in the O-17(p, alpha)N-14 reaction, measured by means of the THM, has been used to renormalize the corresponding resonance strength in the O-17 + p radiative capture channel. As a result, more accurate reaction rates for the O-18(p, alpha)N-15, O-17(p, alpha)N-14, and O-17(p, gamma)F-18 processes have been deduced, devoid of systematic errors due to extrapolation or the electron screening effect. Such rates have been introduced into state-of-the-art red giant branch and asymptotic giant branch (AGB) models for proton-capture nucleosynthesis coupled with extra-mixing episodes. The predicted abundances have been compared with isotopic compositions provided by geochemical analysis of presolar grains. As a result, an improved agreement is found between the models and the isotopic mix of oxide grains of AGB origins, whose composition is the signature of low-temperature proton-capture nucleosynthesis. The low N-14/N-15 found in SiC grains cannot be explained by the revised nuclear reaction rates and remains a serious problem that has not been satisfactorily addressed.