Effect of high-energy resonances on the 18O(P, α) 15N reaction rate at AGB and post-AGB relevant temperatures

Abstract

The 18O(p, α)15 N reaction is of great importance in several astrophysical scenarios, as it influences the production of key isotopes such as 19F, 18O, and 15N. Fluorine is synthesized in the intershell region of asymptotic giant branch (AGB) stars, together with s-elements, by α radiative capture on 15N, which in turn is produced in the 18O proton-induced destruction. Peculiar 18O abundances are observed in R-Coronae Borealis stars, having 16O/18O ≲ 1, hundreds of times smaller than the galactic value. Finally, there is no definite explanation of the 14N/15N ratio in pre-solar grains formed in the outer layers of AGB stars. Again, such an isotopic ratio is influenced by the 18O(p, α)15N reaction. In this work, a high accuracy 18O(p, α)15N reaction rate is proposed, based on the simultaneous fit of direct measurements and of the results of a new Trojan Horse experiment. Indeed, current determinations are uncertain because of the poor knowledge of the resonance parameters of key levels of 19F In particular, we have focused on the study of the broad 660 keV 1/2+ resonance corresponding to the 8.65 MeV level of 19F Since T ∼ 100-300 keV, it determines the low-energy tail of the resonant contribution to the cross section and dominates the cross section at higher energies. Here, we provide a reaction rate that is a factor of two larger above T ∼ 0.5 10 9 K based on our new improved determination of its resonance parameters, which could strongly influence present-day astrophysical model predictions. © 2010. The American Astronomical Society.