Thermonuclear Reaction Rates from Rare Oxygen Isotope Radiative Proton Capture

Matthew Buckner, University of North Carolina, Chapel Hill

At the Laboratory for Experimental Nuclear Astrophysics (LENA), a new resonance strength upper limit has been determined for the 95 keV resonance in the 18O(p,γ)19F reaction. The Electron Cyclotron Resonance Ion Source (ECRIS) and γγ-coincidence detection techniques were exploited to overcome Coulomb suppression and background radiation. At low temperatures relevant to “cool bottom processing” (CBP) in low-mass Asymptotic Giant Branch (AGB) stars, the revised 95 keV resonance strength upper limit was applied to calculate 18O(p,γ)19F reaction rates. We report here on our improved thermonuclear reaction rates and the upper limits we measured for the non-resonant cross section and astrophysical S-factor. The experimental techniques developed for our 18O(p,γ)19F study will be applied to the 17O(p,γ)18F reaction at energies relevant to classical novae. Classical novae are explosive events that occur on the surfaces of accreting white dwarfs in close binary star systems. In these star systems, the parasitic white dwarf leaches hydrogen-rich matter from its main sequence companion. The matter accreted onto the electron-degenerate stellar corpse undergoes periodic thermonuclear runaway at the onset of hydrogen fusion. Classical novae are thought to be a dominant source of the rare oxygen-17 isotope. During a nova event, the 17O(p,γ)18F reaction has an impact on the nova’s oxygen-17 contribution to the interstellar medium, and 17O(p,γ)18F direct-capture cross sections are required to compute reaction rates within the energy range spanning the classical nova Gamow window (0.1 – 0.4 GK). The ECRIS is suited for measuring the direct-capture cross sections within the classical nova Gamow window because it is capable of laboratory beam energies between 50 - 215 keV and average beam currents at the target of I = 1.5 mA. Performing these measurements will improve our understanding of explosive hydrogen burning and the nova contribution to the oxygen-17 abundance in our galaxy. Latest results will be reported.

Abstract Author(s): M. Q. Buckner and C. Iliadis