Multiple populations in Globular Clusters: investigating the uncharted territories with photometry

As established by decades of observational studies, Globular Clusters' (GCs) stars exhibit light-elements variation. Combined spectroscopy, photometry, and modeling efforts revealed that these chemical inhomogeneities arise from GCs harboring multiple stellar populations. While a fraction shares the chemical composition with stars typically found in the Galactic halo, the rest present unexpected abundances of light elements such as helium, carbon, nitrogen, oxygen, and sodium. This latter group of stars has been detected only inside GCs and in no other structure in the Universe. Their origin is nowadays an unresolved riddle of stellar astrophysics. The theoretical framework predicts that the stars with peculiar abundances formed from a gas polluted by massive stars. The several polluters proposed throughout the years have different implications that impact multiple research fields, such as stellar evolution, galaxy assembly, and cosmology. However, none of the proposed scenarios can provide a fulfilling explanation of all the observed features of the multiple populations. Understanding the phenomenon is a mandatory step to shed light on the formation and evolution of GCs and their contribution to building the galaxies and, therefore, shaping the Universe we observe today. Driven by this need, this thesis focuses on revealing new sides of multiple populations by exploiting multi-band photometry -from UV to NIR- taken from several facilities. This work aims to tackle different aspects of this enigma, providing novel constraints for theoretical scenarios. Three main projects constitute the body of my work during the three years of my Ph.D., which are the following: (i) The first focuses on the red Horizontal Branch (HB) and the red clump stars, defining a new set of tools to analyze the phenomenon. UV photometry (both space- and ground-based) is effective in spotting multiple populations among metal-rich GCs in these evolutionary phases and revealing them when Red Giant Branch (RGB) photometry - typically used for this kind of investigation - does not allow a clear separation between chemically different populations. The red HB and red clump stars also allow exploring several aspects of the phenomenon, such as its link with the host galaxy, the radial distribution of the multiple populations, and the chemical inhomogeneities among the stars without peculiar light-elements variations. (ii) Secondly, I explored the Very-Low Mass (VLM) regime of Galactic GCs thanks to Hubble Space Telescope (HST) NIR photometry, detecting in all of them the phenomenon even among these particularly faint stars. For two of the GCs in our sample, the combination of this dataset with optical and UV photometry disentangles chemically different populations among the whole Main Sequence (MS). The properties of the phenomenon do not vary within this stellar mass range, as proven by the lack of differences between their Mass Function (MF) slopes and their constant ratios. These results have implications for constraining the formation scenarios and the nature of the initial MF of the multiple stellar populations. (iii) The third project concerns the Type II GCs, a subclass of clusters that, beyond the light-elements variations, can also exhibit a spread in heavier elements, such as [Fe/H] and s-process elements, and in total C+N+O amount. Here, I show the case of NGC 1851, for which it is performed the most-detailed photometric analysis on Type II GCs yet, defining new tools to study this class of objects. This analysis disentangled four subpopulations and revealed their chemical composition by combining the information from photometry with spectroscopy measurements and stellar spectra. Moreover, the multi-facility approach employed in this work allows the investigation of the spatial and radial distribution of all the populations in NGC 1851 from the center up to beyond the tidal radius.