Multi-Wavelength Study of Hot Stellar Populations in Galactic Globular Clusters

Abstract

Galactic Globular clusters (GGCs) are old stellar systems (age ∼ 12 Gyr) consisting typically of about 104-106 gravitationally bound stars. This makes them excellent astrophysical laboratories to study stellar evolution, particularly at the low-mass end, as well as the role of dynamical processes in the evolution of stars. GGCs are now widely accepted to host stars with variations in light element abundances and/or ages, a phenomenon known as multiple stellar populations (MSPs). The origin of MSPs in GGCs is still an open problem, with several scenarios proposed in the literature. The hot stellar populations (Teff ≳ 5 500 K) in GGCs exist in various advanced, relatively fast-paced stages of low-mass stellar evolution such as horizontal branch (HB; He-core burning phase) including extreme HB (EHB; Teff > 20 000 K), post-horizontal branch (pHB; post-He-core burning phase) and white dwarf (WD). The census and characterization of these stellar populations are crucial to test and improve the models of late evolutionary stages. By studying GGCs with a broad HB color distribution, the MSP phenomenon, especially in terms of helium enhancement, can be probed. In a broader context, the EHB stars and their progeny are speculated to be causing the ‘UV-upturn’ seen in the spectra of elliptical galaxies, making their study highly relevant. Hot stars in GGCs additionally include non-canonical objects such as blue stragglers as well as exotic systems, including cataclysmic variables and other close binaries. The detection and characterization of such sources can shed light on the interplay between stellar evolution and stellar dynamics, in turn, the cluster’s dynamical evolution itself. The main goal of this thesis is to undertake the census and characterization of hot stellar populations in GGCs in order to : (a) shed light on the late stages of lowmass stars’ evolution, (b) identify the formation pathways of non-canonical stars, (c)

trace multiple stellar populations and their origins, and (d) understand the effects of dynamical interactions on stellar evolution. To achieve these aims, a multi-wavelength study of hot stars in three GGCs is conducted. Out of these, two clusters, NGC2808 and ω Centauri (NGC 5139), are massive and dense. Both these clusters have a wide color distribution of HB stars, including a significant population of EHB stars and pHB stars, as well as He-enriched populations. The third cluster is NGC 6397, a low-mass, nearby, core-collapsed cluster ideal for understanding the properties of exotic sources. The thesis mainly uses the ultraviolet (UV) data from the Ultra Violet Imaging Telescope (UVIT) onboard the Indian space observatory, AstroSat. In order to identify the optical counterparts of the UV sources, the published catalogs based on observations from the HST, Gaia, and ground-based telescopes are used. The first complete census and characterization of the pHB population in NGC 2808 revealed that most are AGB-manqu´e stars that have bypassed the asymptotic giant phase altogether after core-He-burning and are expected to progress directly to the WD stage. The UV properties of EHB stars, speculated to be plagued by magnetic spots, are studied. This indicated interesting tentative correlations between their rotational periods and UV magnitudes, though it requires further investigation. The first far- UV photometric study of the most massive GGC ω Centauri helped to disentangle the MSPs along the HB and shed light on the peculiarities in the hot HB and WD populations. The study pointed to the origin of the second-generation stars in the system from the ejecta of intermediate-mass AGB stars and helped achieve constraints on the formation models of ω Centauri in terms of the estimated range in age, [Fe/H], and Y (in particular), for the HB sub-populations. In NGC 6397, exotic BSS binaries, possibly formed through the mass-transfer pathway, are detected. Several He-core WD candidates are also detected in the cluster. This dissertation highlights the importance and advantages of UV observations using facilities such as the AstroSat/UVIT in the studies of hot stellar populations, even in the central regions of massive and dense clusters.