Probing the exotic populations in Globular Clusters: the rotational velocity of Blue Stragglers

Blue Straggler Stars (BSSs) are an "exotic" stellar population observed in Globular Clusters (GCs) and in other stellar environments. BSSs are bluer and brighter than the main sequence turn-off stars, and therefore more massive. Their formation is currently explained by mass transfer activity, direct collisions and merging. Unfortunately, their rotational properties are still largely unconstrained. From a theoretical point of view, high rotational velocities are expected at birth due to angular momentum transfer or conservation, but braking mechanisms should intervene and slow down these stars with unknown timescales and efficiencies. This highlights the need for new observational results able to constrain the BSS formation and evolutionary models. In this PhD Thesis I analyzed high-resolution spectra of a sample of BSSs belonging to the GCs NGC 3201, M55, and NGC 1851 with the aim to characterize their rotational properties. The BSS rotational measurements collected to date revealed the presence of specific trends between the fraction of fast rotating (FR) BSSs (i.e., stars with v sin(i) > 40 km/s) and some structural parameters of the host clusters. In particular, low-concentrated and low-density systems exhibit a higher fraction of FR-BSSs whereas high-concentrated and high-density clusters host significantly smaller percentages of these stars (below 10%), with only a few BSSs rotating faster than 40 km/s. The pieces of evidence collected so far suggest a scenario where recent BSS formation is occurring in low-density environments, likely due to the evolution of primordial binary systems, while it is significantly suppressed in high-density clusters maybe because of a destructive action of stellar collisions on binaries, or a higher efficiency of the intervening braking mechanisms. The results presented in this PhD Thesis can be considered as the starting point for future observational campaigns in other stellar environments and larger samples of stellar clusters.