This study, led by Raphaela Fernandes de Melo, investigates the chemical evolution of key elements like carbon (C), nitrogen (N), oxygen (O), and lithium (Li) within metal-poor stars found in the Milky Way’s halo. By examining 52 giant stars in the red giant branch (RGB) phase, the research aims to understand how the abundance of these elements has evolved from the earliest stars. These “stellar fossils” give scientists insights into nucleosynthesis, or how elements are created in stars, and the chemical pathways in our Galaxy.

Sample and Data Collection

To analyze these elemental abundances, the team selected 52 low-metallicity giant stars (stars with fewer heavy elements than our Sun), avoiding stars with binary companions that could skew the chemical readings. The observations took place at the Very Large Telescope (VLT) using a high-resolution spectrograph, allowing precise measurements of light absorbed by different elements in the stars’ atmospheres. By analyzing specific absorption lines, researchers could determine how much of each element, particularly C, N, O, and Li, was present on the surface of these stars.

Atmospheric Parameters and Abundances

The study calculated various atmospheric parameters, such as effective temperature and surface gravity, to understand each star's evolutionary state. Since stars in the RGB phase undergo mixing—where material from the star's core moves to the surface—the researchers expected these stars to show altered surface compositions. They used a model assuming local thermodynamic equilibrium (LTE) to estimate the surface abundances of C, N, O, and Li, making it possible to distinguish between “mixed” and “unmixed” stars. Mixed stars show the effects of internal processes like the CN-cycle, which converts carbon into nitrogen at high temperatures, while unmixed stars retain the initial chemical composition of the gas cloud from which they formed.

Results and Analysis of Elemental Patterns

The team observed distinct abundance patterns that correlate with the stars’ positions along the RGB. For mixed stars, surface abundances indicated the presence of CN-cycle processed material—carbon levels were lower, and nitrogen levels were elevated. In contrast, unmixed stars displayed higher carbon relative to nitrogen, suggesting these stars had not undergone deep internal mixing. Interestingly, oxygen levels remained mostly unchanged across both mixed and unmixed stars, supporting the idea that the ON-cycle, which would alter oxygen, does not occur under the conditions found in these stars.

Lithium as a Clue for Stellar Evolution

Lithium, created during the Big Bang, is often destroyed in the high temperatures within stars, but it can still be detected in unmixed stars. Nine of the sampled stars retained detectable Li levels, reinforcing their unmixed classification. Mixed stars, however, exhibited no detectable lithium, as the element would have been destroyed through mixing. This relationship between lithium presence and mixing stages provides another useful indicator for studying stellar evolution and internal processes.

Role of Stellar Rotation

The study also considered how stellar rotation impacts element abundances. Rotation enhances mixing, especially in low-metallicity stars, which allows primary nitrogen to be created directly from hydrogen and helium through rotation-driven processes. Galactic chemical evolution (GCE) models that incorporate rotation better match the nitrogen levels observed in unmixed stars, supporting rotation’s role in shaping these elemental patterns. Thus, rotation may be crucial in explaining the chemical signatures seen in ancient stars, potentially offering insights into the evolution of early stars and the Galactic halo.

Conclusions

Through the analysis of C, N, O, and Li abundances, this research highlights how ancient stars in the Milky Way’s halo can provide a record of the Galaxy's early chemical evolution. Unmixed stars, with their original chemical makeup preserved, offer vital clues to the processes in the early Galaxy, while the mixed stars reflect changes due to internal stellar processes. These findings underscore the role of rotation and mixing in shaping stellar abundances and provide valuable constraints for models that aim to explain the formation and evolution of the Galaxy's oldest stars.

Source: Fernandes de Melo