High Resolution Study of Ca40 to Constrain Potassium Nucleosynthesis in NGC 2419

The globular cluster NGC 2419 was the first to exhibit a Mg-K anticorrelation, linked to hydrogen burning at temperatures between 80--260 MK. However, the key K-destroying reaction, ³⁹K(p,γ)⁴⁰Ca, has a large rate uncertainty in this range. We significantly constrain this rate with a high resolution ³⁹K(³He,d)⁴⁰Ca study. We resolve the Eᵣc.m.=154 keV resonance in ³⁹K+p for the first time, increasing the previous rate by up to a factor 13 and reducing its 1σ width by up to a factor of 42. Reaction network calculations for NGC 2419 suggest that this could lower temperatures needed to reproduce the Mg-K anticorrelation.