| Abstract: | Galactic globular clusters (GCs) are old (~12Gyr) and dense stellar environments consisting of thousands to millions of stars. They host almost entire evolutionary phases of old stellar populations and are ideal for studying the formation and evolution history of low-mass stars. The ultraviolet (UV) emission in GCs is mainly dominated by the hot sources of the cluster which are either evolved (e.g., He-burning horizontal branch (HB) stars, post-HB stars, post-AGB stars, AGB-manque stars, and white dwarfs, etc.) or exotic (e.g., blue straggler (BS) and blue hook (BHk) stars) stellar populations that have formed through the dynamical interactions in the dense environment of the cluster. We have studied the UV-bright sources of five Galactic GCs using four far-UV (FUV: 1500 - 2000 A) and three near-UV (NUV: 2000 - 3000 A) filters of the Ultraviolet Imaging Telescope (UVIT) onboard India's first space observatory, AstroSat. The UV/UV-optical colour-magnitude diagrams (CMDs) were constructed to study the evolutionary status of the UV-bright sources of the observed GCs. The HB, hot post-HB, hot BS, and WD stars dominated the FUV-optical CMDs, whereas the NUV-optical CMDs consist of cooler sources (e.g., main-sequence, SGB, RGB, and AGB stars) along with the sources observed in the FUV-optical CMDs. The He abundances of HB stars in three GCs were estimated by matching the observed and model HB stars on the UV and/or UV-optical CMDs, which were found in the range of 0.247−0.350 dex, 0.252−0.265 dex, and 0.247−0.310 dex for NGC 7492, NGC 5272, and NGC 6205, respectively. We could identify several new hot extreme-HB, post-HB, and extremely low mass WD stars, which have either evolved from HB mass less than 0.55 Msun or evolved in a binary system in the dense cluster environment. |
