Proton capture on 17O and its astrophysical implications

Background: The reaction 17O(p,γ)18F influences hydrogen-burning nucleosynthesis in several stellar sites, such as red giants, asymptotic giant branch (AGB) stars, massive stars, and classical novae. In the relevant temperature range for these environments (T9=0.01–0.4), the main contributions to the rate of this reaction are the direct capture process, two low-lying narrow resonances (Er=65.1 and 183 keV) and the low-energy tails of two broad resonances (Er=557 and 677 keV). Purpose: Previous measurements and calculations give contradictory results for the direct capture contribution which in turn increases the uncertainty of the reaction rate. In addition, very few published cross section data exist for the high energy region that might affect the interpretation of the direct capture and the contributions of the broad resonances in the lower energy range. This work aims to address these issues. Method: The reaction cross section was measured in a wide proton energy range (Ec.m.=345–1700 keV) and at several angles (θlab=0∘,45∘,90∘,135∘). The observed primary γ transitions were used as input in an R-matrix code in order to obtain the contribution of the direct capture and the two broad resonances to the low-energy region. Results: The extrapolated S factor from the present data is in good agreement with the existing literature data in the low-energy region. A new reaction rate was calculated from the combined results of this work and literature S-factor determinations. Resonance strengths and branchings are reported for several 18F states. Conclusions: We were able to extrapolate the astrophysical S factor of the reaction 17O(p,γ)18F at low energies from cross section data taken at higher energies. No significant changes in the nucleosynthesis are expected from the newly calculated reaction rate.