Underground study of the O-17(p,gamma) F-18 reaction relevant for explosive hydrogen burning

Background: The O-17( p,gamma)F-18 reaction affects the production of key isotopes ( e.g., F-18 and O-18) in the explosive hydrogen burning that powers classical novae. Under these explosive conditions, the reaction rate is dominated by contributions from a narrow resonance at E-c.m. = 183 keV and by the combined contributions of direct capture and low-energy tails of broad resonances. At present, the astrophysical reaction rate is not well constrained because of the lack of data in the energy region appropriate to classical novae. Purpose: This study aims at the measurement of the O-17( p,gamma)F-18 reaction cross section in order to determine its reaction rate in the temperature region appropriate to explosive hydrogen burning in novae. Method: The O-17( p,gamma)F-18 reaction cross section was measured using both the prompt detection of the emitted gamma rays and an activation technique. Measurements were carried out at the Laboratory for Underground Nuclear Astrophysics ( Gran Sasso, Italy) where the strongly reduced cosmic-ray-induced background allows for improved sensitivity compared to previous studies. Results: The O-17( p,gamma)F-18 reaction cross section was measured in the range E-c.m. = 160 to 370 keV. The strength of the E-c.m. = 183 keV resonance, omega gamma = 1.67 +/- 0.12 mu eV, was determined with unprecedented precision. The total S factor was obtained through a combined fit of prompt gamma-ray and activation results. An overall global fit including other existing data sets was also carried out and a recommended astrophysical reaction rate is presented. Conclusions: The reaction rate uncertainty attained in this work is now below the required precision for nova models. We verified, following a full set of hydrodynamic nova models, that the abundances of oxygen and fluorine isotopes obtained with the present reaction rate are determined with 10% precision and put firmer constraints on observational signatures of novae events.