The Science

Newswise — For the first time, researchers have constrained the rate at which stars convert potassium and hydrogen into calcium through experiments. These reactions are rare in laboratory settings. The researchers therefore used a novel approach: they hitched a proton onto a deuterium nucleus. This maneuver enabled a closer fusion of the proton with potassium, resulting in an amplified signal for measurement. Using these findings, the scientists determined the pace of calcium production in stars, revealing it to be 13 times faster than previously estimated.

The Impact

NGC 2419 is a globular cluster orbiting the Milky Way that is  visible in the constellation Lynx. It contains potassium and magnesium in ratios not found in other, similar globular clusters. Scientists have so far been unable to find the source for this unexpected pattern. However, previous uncertainties in the potassium-hydrogen fusion reactions hindered the predictions of stellar models. This new study offers crucial experimental insights that enable refined stellar models to probe the peculiarities of NGC 2419.


In this study, researchers used the proton stripping 39K(3He,d)40Ca 39K+3He40Ca+2H reaction to populate proton-unbound states in 40Ca at the Triangle Universities Nuclear Laboratory. The high-resolution Enge split-pole spectrograph was used to measure outgoing deuterons coming from 40Ca excited states, and nuclear reaction models were used to extract their quantum mechanical nature.   

The experiment allowed researchers to observe for the first time several states corresponding to proton resonances in the 39K + p system. The researchers then used the states’ energies and proton spectroscopic factors to calculate the rate of the 39K + p 40Ca reaction in stars. The rate was found to be a factor of 13 higher than previous estimates, with uncertainties 42 times smaller. These results mean that the Mg-K anti-correlation could be produced at lower temperatures than previously assumed.



This work is supported by the Department of Energy (DOE) Office of Science, Office of Nuclear Physics. It was performed at the Triangle Universities Nuclear Laboratory, a DOE Center of Excellence.