A JWST Analysis of the Globular Cluster NGC 6528

Multiple stellar populations (MPs) in globular clusters (GCs) challenge the classical view of these systems as simple, single-generation objects and are thought to arise from competing formation channels whose cosmological implications remain debated. In the multiple-generation (pollution) scenario, GCs would have been substantially more massive in the past, potentially playing a major role in the reionization of the Universe through intense early star formation and feedback. In contrast, the Bondi-like accretion scenario does not require such extreme initial masses. A decisive test between these scenarios is provided by the oxygen abundance in M-dwarf stars, which preserve their pristine birth composition. An oxygen spread in the lower MS similar to that derived spectroscopically on the RGB would strongly favor the multiple-generation scenario, whereas a significantly lower value would support accretion. This thesis presents the first deep photometric investigation of MPs in the metal-rich bulge GC NGC 6528, combining HST and JWST/NIRCam observations to reach below the MS knee for the first time in this cluster. After a membership selection based on relative HST-JWST proper motions (PMs) and differential reddening corrections, the near-infrared CMDs reveal clear MS broadening. The helium-sensitive F115W-F200W color yields a helium abundance spread of ΔY~0.09, while [O/Fe] variations are constrained to ~0.29 dex. The derived oxygen variation is compatible with the value of ~0.2 dex measured indipendently with spectroscopy in previous studies. This agreement strongly supports the multiple-generation scenario for NGC 6528. These results emphasize the enormous potential of JWST for unveiling MPs in extreme environments and offer a proven methodological framework for systematic exploration of the poorly studied metal-rich bulge GC population.