Globular clusters are collections of hundreds of thousands of stars. They are thought to have evolved completely isolated from the rest of the universe. As such, they are perfect “stellar laboratories” where scientists can test their theories of the origin of the elements. Despite their seemingly simple evolution, these stars’ chemical makeup is far from easy to understand. In particular, current understanding in astrophysics theories do not fully explain the amount of two elements—sodium and oxygen—measured in stars in globular clusters. This research turned to the nuclear physics of an important type of reaction involving the sodium in these stars to help unlock their mysteries.
One of the main goals of nuclear astrophysicists is to understand the origin of elements. The Big Bang made mostly hydrogen and helium. How did all other elements needed to make life possible come into existence? Although scientists know that many generations of stars slowly built what we now see in the solar system, the details are elusive. The goal of this research was to measure the nuclear reactions that happen in those stars. In doing so, scientists gain a deeper understanding of what happened in their interiors to create the solar system as we know it today.
Reliable stellar models to explain observations of sodium in globular clusters are difficult for scientists to develop. In part, this is because the rates that nuclear reactions occur in the stars are not well known. One factor responsible for those rates are proton capture reactions on sodium-23. These reactions depend on low-energy resonances that are exceedingly difficult to measure. This research used a proton-stripping reaction to probe those resonances with the high-resolution Enge split-pole spectrograph at the Triangle Universities Nuclear Laboratory, a Department of Energy (DOE) Nuclear Physics Center of Excellence.
In measuring the proton stripping reaction of helium-3 on sodium-23, the researchers developed unique analysis methods using Bayesian statistical techniques. This allowed them to find the energy of the key resonances in the sodium-23 proton capture reactions that affect sodium production in globular clusters. The scientists found, critically, that one resonance is much lower in energy than previously predicted. This increases the rate at which sodium is destroyed in stars by a factor of two. Stellar models using these findings predict less sodium production in globular clusters, possibly necessitating the need to rethink cluster evolution models.
This research was supported by the Department of Energy Office of Science, Office of Nuclear Physics.