Dynamical Evolution of Dense Stellar Systems from Blue Straggler Stars

The aim of this Thesis is to investigate the dynamical evolution of Globular Clusters (GCs) by exploiting the properties of a dynamically-sensitive stellar population: Blue Straggler Stars (BSSs). These objects are ∼3 times more massive than the average in GCs, and their presence cannot be explained in terms of the standard evolution of single stars. This Thesis provides the determination of the physical properties (mass, temperature, radius, surface gravity) of BSSs along the entire extension of their sequence in the color-magnitude diagram, a study that confirms the large mass of these objects. Being more massive than the average, BSSs significantly suffer from the effects of dynamical friction, which makes them sink toward the cluster center. We demonstrated that the selection of BSSs at ultraviolet wavelengths guarantees the highest achievable level of completeness, and we used this approach to determine the BSS radial distribution in ∼30% of the entire Milky Way GC population. This allowed us to rank the host clusters in terms of the level of their internal dynamical evolution. The same approach applied to five GCs in the Large Magellanic Cloud not only demonstrated that the BSS radial distribution is a powerful dynamical indicator even in external galaxies, but also allowed us to show that the spread in core radius observed for the old GCs in the Large Magellanic Cloud is just the natural consequence of their internal dynamical evolution. We also pursued the search for double BSS sequence in M15 and NGC 1261. In M15, we found evidence for two distinct sequences of collisional BSSs that likely testify the occurrence of the core collapse and a subsequent core oscillation. Instead, in NGC 1261 we found only a hint of a double BSS sequence, the detectability of which is not as solid as for the other reported cases