Exploiting globular clusters to shed light on the early Galaxy

Globular clusters (GCs) are ancient stellar systems that formed from the densest regions of supergiant molecular clouds during the early stages of galaxy formation. Understanding their formation and chemical composition is challenging, particularly because the intra-cluster medium in present-day GCs is minimal and likely contaminated by material ejected by massive stars during their post-main-sequence (MS) evolution. Furthermore, GCs host multiple stellar populations (MPs), adding complexity to inferring the chemical makeup of the original gas. GCs contain two main populations of stars: first-generation (1G) stars, which retain the chemical composition of the original gas, and second-generation (2G) stars, which show enhanced helium, nitrogen, and sodium and depleted carbon and oxygen. Studying 1G stars offers a unique opportunity to trace the chemical properties of the primordial clouds from which GCs formed. Recently, the assumption of chemical homogeneity in 1G stars has been challenged, with the Chromosome Map (ChM), a photometric tool revealing chemical variations, showing that 1G stars often display extended or bimodal sequences, suggesting previously unrecognized chemical diversity. These variations could stem from factors such as unresolved binaries or intrinsic differences in helium and metallicity. My Ph.D. research focuses on three key projects that explore this phenomenon further: (i) Differential reddening broadens evolutionary sequences in color-magnitude diagrams (CMDs), making it difficult to accurately identify 1G stars in GCs. I used data from the Hubble Space Telescope UV Legacy Survey to estimate local reddening variations in 56 Galactic GCs. After correcting for differential reddening, CMDs were significantly improved for 21 GCs. Additionally, I measured the RV parameter, which shapes the Galactic reddening law, and found substantial variability in RV values across the Galaxy, ranging from ~ 2.0 to ~ 4.0. These refined values were used to create high-resolution reddening maps for the most reddened clusters, which were made publicly available. (ii) In my second project, I investigated chemical variations among unevolved MS stars in the GCs NGC 6362 and NGC 6838. Like red-giant branch stars, 1G MS stars showed broader ChM sequences than 2G stars, indicating chemical inhomogeneities in the original gas. I developed a new photometric diagram to separate stellar populations based on iron content, revealing that star-to-star metallicity variations are the primary drivers of the extended 1G sequence, while unresolved binaries play a minimal role. These findings were extended to 55 GCs, where the metallicity spread within 1G stars ranged from ~ 0.05 to ~ 0.30 dex and showed a mild correlation with cluster mass and overall metallicity. This provides constraints on MP formation scenarios, suggesting multiple stellar generations. (iii) The third project examined whether the extended 1G sequence is unique to GCs with MPs. I analyzed the Galactic open cluster NGC 6791 and the Large Magellanic Cloud cluster NGC 1783, both simple-population clusters, and found extended color sequences linked to metallicity variations, similar to those in Galactic GCs. Additionally, I explored the relationship between metallicity variations and radial distance from the cluster center, revealing a metallicity radial gradient in 1G stars in the Galactic GC 47 Tucanae. Finally, I investigated whether 1G metallicity distributions correlate with host cluster parameters but found no significant connections, highlighting the complex nature of 1G stars. These projects offer new insights into the chemical diversity of 1G stars in GCs, helping to better understand their formation environments and the processes that shaped these ancient stellar systems.