The Galactic Center, home to the Nuclear Star Cluster (NSC), is a densely packed region of stars that offers clues about the Milky Way's formation and evolution. By studying chemical elements like magnesium (Mg), silicon (Si), and calcium (Ca) in stars, researchers can reconstruct the history of star formation and gas dynamics in this central region. Nils Ryde and colleagues conducted a study of nine red giant stars in the NSC to understand their chemical compositions and how these compare with other regions of the galaxy.

Background

The NSC, a spherical group of stars at the Milky Way's center, is surrounded by structures like the Nuclear Stellar Disk and the Central Molecular Zone. These areas host stars of different ages, with some as old as 10 billion years. Two main theories explain the NSC's formation: in-situ star formation triggered by gas inflows or the merging of massive star clusters.

Chemical "fingerprints" in stars, such as the ratios of α-elements (like Mg, Si, and Ca) to iron (Fe), reveal the star formation rates and timescales. α-elements are typically produced in massive stars and released during supernova explosions, while iron builds up more slowly through other processes. Therefore, studying these ratios helps identify whether star formation happened quickly or over long periods.

Methods

The team analyzed nine M-type giant stars, selected for their brightness and location within the NSC. High-resolution infrared spectra were collected using the IGRINS spectrograph at the Gemini South telescope. These observations were compared to a control group of 50 stars from the solar neighborhood. To minimize errors, the team used a consistent method across both samples, ensuring the results reflected real differences rather than measurement inconsistencies.

Results

The study revealed clear patterns in the α-element trends:

• Magnesium (Mg): The abundance of Mg relative to Fe decreased with increasing metallicity. NSC stars followed a trend like stars in the Galactic bulge and thick disk, suggesting rapid star formation in the NSC's past.

• Silicon (Si): Si also showed a decreasing trend with metallicity, consistent with the Mg findings. This similarity indicates shared formation processes.

• Calcium (Ca): The Ca trends were slightly more variable but still followed a general decline with metallicity.

When combining all three α-elements, the team observed a steady decrease in their abundance ratios as metallicity increased. This trend matches patterns found in the inner regions of the Milky Way, reinforcing the connection between the NSC and the Galactic bulge.

Discussion

These results support the idea that the NSC shares a similar evolutionary history with the Galactic bulge, characterized by rapid star formation and enrichment of α-elements. This challenges theories suggesting the NSC experienced a dominant burst of star formation in the recent past. Instead, the findings align with a scenario where the NSC's stars formed in a series of events spread over billions of years, mirroring processes in the thick disk and inner bulge.

Conclusion

The study provides new insights into the NSC's chemical makeup and its links to the broader Galactic environment. By showing that the NSC follows trends seen in other parts of the Milky Way, the research highlights the interconnected history of different Galactic structures. Future surveys with larger sample sizes, like those planned with the MOONS instrument, promise to refine this understanding further.

Source: Ryde