Measurement of the Erc.m.=138 keV resonance in the 23Na(p,γ)24Mg reaction and the abundance of sodium in AGB stars

Globular clusters represent some of the oldest stellar aggregations in the universe. As such, they are used as testing grounds for theories of stellar evolution and nucleosynthesis. Astronomical observations have shown star-to-star abundance variations in light-mass elements in all galactic globular clusters that are not predicted by standard stellar evolution models. In particular, there exists a pronounced anticorrelation between Na and O in the cluster stars that is not observed in field stars of similar evolutionary state. The abundance of Na is regulated in part by the Na-23+p reaction, which is also a bridge between the NeNa and the MgAl mass regions, but the Na-23(p,gamma)Mg-24 reaction rate is very uncertain for burning temperatures relevant to stars on the red giant and asymptotic giant branches. This uncertainty arises from an expected but unobserved resonance at E-r(c.m.) = 138 keV. The resonance strength upper limit has been determined to be omega gamma(UL)(138 keV) <= 5.17 x 10(-9) eV with indications of a signal at the 90% confidence level. New reaction rates have been calculated for the Na-23(p,gamma)Mg-24 and Na-23(p,alpha)Ne-20 reactions and the recommended value for the Na-23(p,gamma)Mg-24 rate has been reduced by over an order of magnitude at T-9 = 0.07. This will have implications for the processing of material between the NeNa and MgAl mass regions.