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, ${}^{39}\mathrm{K}(p,\gamma ){}^{40}\mathrm{Ca}$, has a large rate uncertainty in this range. We significantly constrain this rate with a high resolution ${}^{39}\mathrm{K}({}^3\mathrm{He},d){}^{40}\mathrm{Ca}$ study. We resolve the ${\mathrm{E}}_{\mathrm{r}}^{\mathrm{c}.\mathrm{m}.}=154\mathrm{keV}$ resonance in ${}^{39}\mathrm{K}+p$ for the first time, increasing the previous rate by up to a factor 13 and reducing its $1\sigma$ 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.

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• Nuclear astrophysics
• Nuclear reaction rates
• Transfer reactions
• Stars
• 39 ≤ A ≤ 58