UV stellar populations in globular clusters: horizontal branch morphology and blue straggler stars

Abstract

Globular Clusters (GCs) are the systems that contains million of stars bound together by gravity forming a spherical distribution. As the timescales of dynamical encounters occurring in these dense systems are shorter than the age of the universe, they serve as ideal laboratories to study the formation and properties of exotic interacting stellar systems such as Blue Straggler Stars (BSSs), X-ray binaries etc. The Ultra-Violet (UV) observations of Galactic GCs (GGCs) in the past decade with HST and GALEX has uncovered interesting results such as detection of BSSs and very hot horizontal branch (HB) stars like Extreme horizontal branch (EHB) and blue hook (BHk) stars, multi- ple stellar populations (MSPs) in the color-magnitude diagrams (CMDs) etc. These results have shed new light in our understanding of the stellar evolution and dynamics in GCs. In this thesis work, we adopted UV approach to study the hot stellar populations in three GCs: NGC 1851, NGC 288 and NGC 5466. We used Ultra-Violet Imaging Telescope (UVIT) data onboard Indian space observatory, AstroSat to obtain the Far- UV and Near-UV observations of the clusters. To identify the clusters members, we used the proper motion data of Gaia DR2. By combining UVIT with HST for the central regions and with ground observations for the outer regions, we generated the UV and UV-optical CMDs for the entire cluster region. These combinations along with the model isochrones made it possible to identify the locations of the hot stellar populations and classify them in UV CMDs. The UV study on the HB morphology of the cluster NGC 288 unravels important results such as two peaks in the temperature distribution, the presence of several gaps in the HB, identification of three EHB candidates, etc. An important result is the deviation of the observed HB in the FUV CMDs from the isochrone at temperature 11,500 K suggesting an onset of atomic diffusion. Similarly, our study on the HB population in a massive GC NGC 1851 using UV-optical CMDs reveals that the cluster hosts at least two stellar populations with either an age difference of 2 Gyr or an initial helium difference of 0.04 dex with a similar metallicity. We also studied the UV variability of the RR Lyrae stars and detected three new such candidates in this cluster. Our study on the cumulative radial distribution of BSSs with respect to the HB stars in two low density GCs NGC 288 and NGC 5466, suggests that the BSSs are more centrally concentrated than HB stars. A study of the specific frequency of BSSs as a function of radius suggest that the clusters are of intermediate dynamical age, which in turn indicates that the dynamical friction has affected only up to a certain radius (1.5 times the half-light radius of the clusters) causing the segregation of massive BSSs. These clusters were found to have the highest specific frequency of BSSs known among the GGCs. Since stellar collisions are not that effective to create a BSS in these low- density clusters, the mass transfer in binaries might have led to such high rate of formation. This is supported by the detection of a hot white dwarf (WD) companion to a BSS located in the outskirts of cluster NGC 5466 which is described in this work. Thus, our study demonstrates the capability of UVIT in resolving the stars in the centre of the clusters for example NGC 288 and NGC 5466. With its superior resolution and large field of view, UVIT provides the best platform for the sample selection of BSSs and HB stars to study their radial distribution in the entire cluster region. In addition, its multiple filters are useful for constructing the SEDs and estimating the parameters of the hot stellar populations.