Direct Capture Cross Section and the Ep=71 and 105 keV Resonances in the Ne 22 (p,γ) Na 23 Reaction

The Ne-22(p,gamma)Na-23 reaction, part of the neon-sodium cycle of hydrogen burning, may explain the observed anticorrelation between sodium and oxygen abundances in globular cluster stars. Its rate is controlled by a number of low-energy resonances and a slowly varying nonresonant component. Three new resonances at E-p = 156.2, 189.5, and 259.7 keV have recently been observed and confirmed. However, significant uncertainty on the reaction rate remains due to the nonresonant process and to two suggested resonances at E-p = 71 and 105 keV. Here, new Ne-22(p,gamma)Na-23 data with high statistics and low background are reported. Stringent upper limits of 6 x 10(-11) and 7 x 10(-11) eV (90% confidence level), respectively, are placed on the two suggested resonances. In addition, the off-resonant S factor has been measured at unprecedented low energy, constraining the contributions from a subthreshold resonance and the direct capture process. As a result, at a temperature of 0.1 GK the error bar of the Ne-22(p,gamma)Na-23 rate is now reduced by 3 orders of magnitude.