16O(p,g)17F Reaction and the C and O Isotopic Ratios in Red Giant Branch Stars

The 16O∶17O isotopic ratio is a widely used tracer of nucleosynthesis and mixing processes in red giant stars. Its value depends on the rates of the proton-capture reactions responsible for the production and destruction of 16O and 17O among which the 16Oðp; γÞ17F reaction plays a central role, as it involves both isotopes. This work reports on a new measurement of the 16Oðp; γÞ17F cross section carried out at the Laboratory for Underground Nuclear Astrophysics (LUNA) over a center-of-mass energy range of 200– 380 keV, a region previously unexplored by experimental studies. Two complementary techniques were employed: prompt gamma-ray detection and activation. The results obtained with the two techniques are consistent within experimental uncertainties. From the measured cross sections and existing literature data, we evaluate a new thermonuclear reaction rate, which is then used to demonstrate how simultaneous measurements of stellar carbon and oxygen isotopic ratios constrain the initial mass and the extent of the mixing events in red giant branch stars. In particular, we demonstrate how the 12C:13C abundance ratio is primarily sensitive to the temperature Tmax attained during mixing, whereas the 16O:17O abundance ratio is highly sensitive to the initial stellar massMi. Therefore, simultaneous measurements of these isotopic ratios can disentangle and place stringent constraints on Mi and Tmax, and the remaining uncertainty is now dominated by stellar abundance measurements rather than nuclear-physics inputs. [LUNA Collaboration]