The Blue Straggler Star Populations in Galactic Globular Clusters

The puzzling existence of Blue Straggler Stars (BSSs) implies that they must form in relatively recent events, after the majority of the constituent globular cluster (GC) stellar population was formed. In this thesis we compile a large set of independent work to help understand the formation of BSSs. In Chapter 2 we present new proper-motion cleaned BSS catalogs in 38 Milky Way GCs based on multi-passband and multi-epoch treasury survey data from the Hubble Space Telescope. We study the accuracy of our proper motion measurements using estimates of central velocity dispersions and find very good agreement with previous studies in the literature. Finally, we present a homogeneous BSS selection method, that expands the classic BSS selection parameter space to more evolved BSS evolutionary stages. In Chapter 3 we present the largest ever data set of velocity dispersion profiles of BSSs, which confirm that BSSs are populations that have been greatly affected by two-body relaxation, as it has been previously expected based on basic equipartition principles. In addition, we show from photometric BSS mass estimates that there is a striking correlation between the average BSS mass and the cluster dynamical age. We relate the measured proper-motion velocity dispersions with the photometric BSS mass estimates and use them to propose a novel interpretation of BSS CMDs and its relationship with the binary-evolution and dynamical formation channels. In Chapter 4 we present the first dynamical study of BSSs in three galactic GCs: NGC 3201, NGC 5139 (ωCen), and NGC 6218. We measure the v sin(i) values of the sample BSSs and find their distribution functions peaked at slow velocities with a long tail towards fast velocities in each globular cluster. We find that the BSSs in each of the GC sample which show vsin(i)>70 km s−1 are all found in the central cluster regions. In all globular clusters we find rapidly rotating BSSs that have relatively high differential radial velocities which likely put them on hyperbolic orbits, suggestive of strong dynamical interactions in the past. In Chapter 5 we present a multi-passband photometric study of the BSS population in NGC 1261. The inner BSS population is found to have two distinct sequences in the color-magnitude diagram, similar to double BSS sequences detected in other GCs. These well defined sequences are presumably linked to single short-lived events such as core collapse, which are expected to boost the formation of BSSs. In agreement with this, we find a BSS sequence in NGC 1261 which can be well reproduced individually by a theoretical model prediction of a 2 Gyr old population of stellar collision products, which are expected to form in the denser inner regions during short-lived core contraction phases. We argue that NGC 1261 can be considered a candidate for the post-core-collapse bounce state seen in dynamical simulations of old GCs