New measurements of low-energy resonances in the Ne 22 (p,γ) Na 23 reaction

The Ne22(p,γ)Na23 reaction is one of the most uncertain reactions in the NeNa cycle and plays a crucial role in the creation of Na23, the only stable Na isotope. Uncertainties in the low-energy rates of this and other reactions in the NeNa cycle lead to ambiguities in the nucleosynthesis predicted from models of thermally pulsing asymptotic giant branch (AGB) stars. This in turn complicates the interpretation of anomalous Na-O trends in globular cluster evolutionary scenarios. Previous studies of the Ne22(p,γ)Na23, Ne22(He3,d)Na23, and C12(C12,p)Na23 reactions disagree on the strengths, spins, and parities of low-energy resonances in Na23 and the direct-capture Ne22(p,γ)Na23 reaction rate contains large uncertainties as well. In this work we present new measurements of resonances at Erc.m.=417, 178, and 151 keV and of the direct-capture process in the Ne22(p,γ)Na23 reaction. The resulting total Ne22(p,γ)Na23 rate is approximately a factor of 20 higher than the rate listed in a recent compilation at temperatures relevant to hot-bottom burning in AGB stars. Although our rate is close to that derived from a recent Ne22(p,γ)Na23 measurement by Cavanna et al. in 2015, we find that this large rate increase results in only a modest 18% increase in the Na23 abundance predicted from a 5 M thermally pulsing AGB star model from Ventura and D'Antona (2005). The estimated astrophysical impact of this rate increase is in marked contrast to the factor of ∼3 increase in Na23 abundance predicted by Cavanna et al. and is attributed to the interplay between the Na23(p,α)Ne20 and Ne20(p,γ)Na21 reactions, both of which remain fairly uncertain at the relevant temperature range

High-intensity-beam study of O 17 (p,γ) F 18 and thermonuclear reaction rates for O 17 +p

Hydrogen burning of the oxygen isotopes takes place in low-mass stars, asymptotic giant branch stars, and classical novae. Observations of oxygen elemental and isotopic abundances in stellar spectra or in presolar grains provide strong constraints for stellar models if reliable thermonuclear reaction rates for hydrogen burning of oxygen are available. We present the results of a new measurement of the 17O(p,γ)18F reaction in the laboratory bombarding energy range of 170-530 keV. The measurement is performed with significantly higher beam intensities (Imax ≈ 2 mA) compared to previous work and by employing a sophisticated γ-ray coincidence spectrometer. We measured the cross section at much lower energies than previous in-beam experiments. We also apply a novel data-analysis technique that is based on the decomposition of different contributions to the measured pulse-height spectrum. Our measured strengths of the low-energy resonances amount to ωγpres(193keV)=(1.86±0.13)×10-6 eV and ωγpres(518keV)=(13.70±0.96)×10-3 eV. For the direct capture S factor at zero energy, we find a value of SDCpres(0) = 4.82±0.41 keV b. We also present new thermonuclear rates for the 17O+p reactions, taking into account all consistent results from previous measurements