A Tenuous Signal: Searching for Heavy Element Dispersion in the Stars of M5

Nalamwar and collaborators investigate how some of the heaviest elements in the universe formed by studying stars in the globular cluster M5. Globular clusters are dense groups of ancient stars orbiting the Milky Way, and M5 is one of the brighter and better-studied examples. This study focuses on the so-called r-process, the “rapid neutron capture” mechanism that creates heavy elements like gold and uranium. While events like neutron star mergers are known r-process sources, their rarity raises questions about whether they can explain all the heavy elements we observe in old stars. Globular clusters, with their multiple generations of stars, provide a unique way to test these ideas.

The Science Question

The r-process requires extreme environments with huge numbers of free neutrons. Astronomers know these conditions can occur in neutron star mergers, and possibly in rare types of supernovae. But where the r-process happened early in the Milky Way’s history is still debated. Previous studies of clusters such as M15 and M92 showed that different stars can have slightly different amounts of r-process elements, hinting that the cluster’s original gas was not fully mixed. Nalamwar and colleagues set out to see if a similar dispersion exists in M5, which is somewhat more metal-rich than the previously studied clusters.

Observing M5’s Stars

The team used archival spectra from the Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES). They focused on 28 red giant stars (the older, swollen descendants of sun-like stars) because these stars are bright enough to give clear spectral lines. By measuring absorption features of barium (Ba), neodymium (Nd), and europium (Eu), the authors traced whether these elements varied from star to star. They also classified stars into “first” and “second” generations based on sodium (Na) and oxygen (O) abundances, since globular clusters are known to host multiple stellar populations.

A Hint of Dispersion

The analysis revealed something interesting: neodymium showed a small but measurable spread in the first generation of stars, while europium did not. In statistical terms, the first-generation Nd dispersion was about 0.15 dex, which is just above a 2-sigma detection. For the second generation, no significant spread was seen. Barium, mostly produced by a different process (the “s-process”), also showed no strong variations. This means that if M5 did inherit uneven r-process enrichment, it likely happened only before or during the formation of the first generation of stars.

What This Means for the r-process

Two scenarios could explain the result. In one, a rare but powerful r-process event, like a magneto-rotational supernova or even a very early neutron star merger, polluted the gas cloud that formed the first generation. In the other, M5 may have formed from the merger of different gas clouds, each carrying slightly different r-process signatures. Either way, the lack of dispersion in the second generation suggests the gas was well-mixed by the time those stars formed.

The Bigger Picture

The findings for M5 are more subtle than the clear dispersions found in clusters like M15 and M92. Still, they hint that r-process enrichment in globular clusters might not be uniform across all environments. A single outlier star, unusually poor in Nd but not Eu, highlights the complexity of the problem and the need for more detailed studies. The authors stress that their conclusions are limited by the quality of older Keck data, which had fewer measurable lines for some elements. Future studies targeting stars with very similar temperatures and gravities may sharpen the picture.

Source: Nalamwar