High Resolution Study of ⁴⁰Ca to Constrain Globular Cluster NGC 2419

Globular clusters exhibit abundance correlations between light-element pairs in their low-mass stars. These correlations result from mixing with the pollution of older stars in the cluster. The Mg-K anticorrelation among red giants in the cluster NGC 2419 is an intriguing example due to its strong K enrichment. The enrichment mechanism stems from hydrogen burning in the polluter stars at temperatures between 80–260 MK. However, the key potassium-destroying reaction, ³⁹K(p, γ)⁴⁰Ca, has a large reaction rate uncertainty at these temperatures. We significantly constrain its reaction rate with a high resolution ³⁹K(³He, d)⁴⁰Ca study using the Enge Split-Pole Spectrograph at the Triangle Universities Nuclear Laboratory. We resolve the 154 keV resonance in ³⁹K+p for the first time, leading to an increase in the previously calculated rate by up to a factor of 13 and a reduction in its 1σ width by up to a factor of 42. Using reaction network calculations, we demonstrate the effect this new rate has on the T-ρ conditions that reproduce the observed Mg-K anticorrelation.