How Giant Stars at Low Metallicity Shape the Chemistry of the Early Universe

Globular clusters are ancient groups of stars that often show strange chemical patterns not seen in other parts of the galaxy. One of the most famous of these is the sodium-oxygen (Na-O) anti-correlation, where some stars in a cluster have high sodium and low oxygen, while others are the opposite. This pattern suggests that some stars formed from material already processed by earlier generations. Higgins et al. explore whether very massive stars (VMS), especially those in low-metallicity environments like the early Universe, could have produced these chemical signatures through their stellar winds.

Modeling the Lives of Massive Stars

To investigate this, the authors use detailed computer models to simulate how stars with initial masses from 30 to 500 times that of the Sun evolve and lose mass. They focus on stars in environments with metallicities as low as 1% that of the Sun. The models include different phases of a star’s life—hydrogen burning and helium burning—and account for both non-rotating and rotating stars. Since metallicity affects how easily a star can lose mass through winds, the study tests how these winds behave in such extreme, low-metal environments.

What the Models Reveal About Nucleosynthesis

As stars burn hydrogen and helium in their cores, they create new elements through nuclear reactions—a process called nucleosynthesis. The models show that lower metallicity stars reach higher central temperatures, which changes how certain elements form and are destroyed. Notably, sodium (Na) and aluminum (Al) are particularly sensitive to these temperature changes. Even though the total amount of mass lost by stellar winds is smaller at low metallicity, these winds still carry away large amounts of chemically enriched material, especially in stars between 80 and 300 solar masses.

Rotation Makes a Difference

Adding rotation to the models changes things further. Rotating stars mix their inner layers more efficiently, which brings newly formed elements like nitrogen (N) and sodium to the surface faster. This leads to stronger chemical enrichment in the winds. However, the effect of rotation is most pronounced in stars below about 100 solar masses. At higher masses, even rapid rotation does not change the outcome much because these stars lose angular momentum quickly through winds.

Connecting Theory to Observations of Globular Clusters

The authors compare their model results with real observations of stars in globular clusters like NGC 5904. They find that very massive stars—particularly those over 200 solar masses—can produce the right mix of Na-rich and O-poor material to explain the second population of stars in these clusters. These stars shed their outer layers early, releasing the processed elements into the cluster environment. Lower-mass stars don't match the observed abundance patterns as well, since they do not expose and eject enough altered material during their lifetimes.

The Role of Dilution and Other Element Patterns

To match the chemical signatures seen in globular clusters, the enriched material from massive stars likely mixed with unprocessed gas before forming new stars. The authors test various dilution levels in their models and find that only a small amount of mixing is needed to reproduce the observed Na-O patterns. They also explore the magnesium-aluminum (Mg-Al) anti-correlation, another observed signature in some clusters, but find it harder to match. This may point to unknown physics or the need for different sources or reaction rates than currently modeled.

Conclusion: VMS Winds as Key Contributors to Early Chemical Enrichment

This study highlights the significant role that very massive stars at low metallicity may have played in shaping the chemical evolution of the early Universe. Even with weaker winds, these stars can still eject enough processed material to influence the next generation of stars—especially in dense environments like globular clusters. By demonstrating how these winds affect nucleosynthesis and elemental yields, the paper supports the idea that VMS are a compelling explanation for the peculiar chemistry seen in ancient star clusters.

Source: Higgins