Binaries among multiple populations in the globular cluster 47 Tucanae

Nearly all globular clusters (GCs) host multiple stellar populations composed of stars with different helium and light-element abundances. These comprise first-population stars, which share the same chemical composition as halo-field stars with similar [Fe/H], and the second populations, which are composed of stars with enhanced helium and nitrogen abundances and depleted content of oxygen and carbon. Each stellar population delineates distinct sequences in certain photometric diagrams, which can disentangle stars with different chemical compositions at different evolutionary stages. Nowadays, one of the most intriguing open questions regarding GCs pertains to the formation and evolution of their multiple populations. Recent works based on N-body simulations of GCs show that the fractions and characteristics of binary stars can serve as dynamic indicators of the formation period of multiple-population GCs and their subsequent dynamical evolution. Nevertheless, the incidence of binaries among multiple populations is still poorly studied. Moreover, the few available observational studies are focused on bright stars of a few GCs alone. In this master thesis, I used deep images of the GC 47 Tucanae collected with the Hubble Space Telescope (HST) to investigate, for the first time, the incidence of binaries among multiple populations of M-dwarfs. To reach this objective, I follow a new approach, which is based on observations of binaries in the F606W, F814W, F110W, and F160W filters of HST and grids of simulated binaries. I find that in the analyzed field of view, where the first population comprises ∼40% of the total number of stars, the majority (87±13%) of the studied binaries are composed of first population stars. These findings are consistent with the scenarios where the second-population stars form in the cluster center, arranging in very-dense and compact stellar groups embedded in a more extended first-population system.