Investigation of the Red Giant Branch Bump in Globular Clusters

In this thesis, we present a detailed photometric analysis of the Red Giant Branch Bump in a sample of globular clusters using the data from Hubble Space Telescope. The main objective is to understand how the properties of the RGB bump, such as its magnitude, color, and strength, are influenced by the cluster’s metallicity and other stellar population characteristics. First, we processed and cleaned the data to build accurate color–magnitude diagrams. We then identi- fied the RGB bump in each cluster using the luminosity function and measured its strength. From the CMDs, the key photometric parameters including magnitude and color separations between the Main Sequence Turn-Off, RGB Bump, and Red Horizontal Branch, were ob- tained. Among the clusters, Terzan 5 is notable for its complex stellar population and the pres- ence of two RHB. For this cluster, the RHB-RGBB magnitude difference was measured by combining both clumps. We compared clusters in the Galactic bulge with clusters outside the bulge to understand how the RGB bump changes with metallicity. A clear dependence of RGB bump properties on metallicity is observed. The color and magnitude separations between MSTO and RGBB de- crease with metallicity [Fe/H], consistent with predictions from stellar models. The magnitude difference between the RHB and RGBB increases with metallicity, indicating that the RGB bump becomes fainter relative to the RHB in more metal-rich clusters. The normalized RGB bump strength shows only a weak dependence on metallicity but also indicates differences be- tween the two cluster groups. We obtained theoritical iscohrone models from the BaSTI and their comparisons with the ob- served data suggest that age variations alone cannot explain the observed differences between the two cluster samples. Instead, the results indicate that helium enrichment plays a significant role, particularly for bulge clusters, which are better reproduced by helium-enhanced models. Overall, this study shows that the RGB bump can be used to investigate stellar population properties in globular clusters.