Study of UV-bright Stars in Galactic Globular Clusters Using Ultraviolet Imaging Telescope (UVIT) Observations

Abstract

The hot stellar systems of a globular cluster (GC), such as horizontal branch (HB) stars, hot post-HB stars evolving towards asymptotic giant branch (AGB) phase, AGB-manqué stars, post-AGB stars, post-(early) AGB stars, white dwarfs (WDs), blue straggler (BS) stars, and blue-hook (BHk) stars, among others, dominate the ultraviolet (UV) emission of the cluster. UV imaging of GCs enable us to determine the evolutionary status of these hot sources and to study the dynamics of the GCs based on the quantity and nature of such hot stellar systems present in the cluster. We present the study of five Galactic GCs, NGC 4147, NGC 7492, NGC 4590, NGC 5272, and NGC 6205 using UV imaging observations with Ultraviolet Imaging Telescope (UVIT) onboard Indian space observatory satellite AstroSat. The UVIT observations were performed in several far-UV (FUV: 1300−1800 Å) and near-UV (NUV: 2000−3000 Å) filters. Apart from the UV observations, several publicly available archival catalogs of GCs such as Hubble Space Telescope (HST) GC survey, Gaia catalog of GCs, Ground-based photometry of GCs, Pan-STARRS catalog, 2MASS catalog, etc., were also used to get the optical and infra-red counterparts of the UVIT observed cluster member stars. The UV and UV-optical color-magnitude diagrams (CMDs) were constructed using UVIT FUV and NUV filters along with optical Gaia G, HST F606W, and V filters to identify the stellar evolutionary phases of the hot UV-bright sources present in the cluster. We identified a total of 1027 HB stars, 34 post HB stars (a mixture of AGB-manqué, post (early) AGB, and hot post-HB stars evolving towards AGB phase), 55 BS stars, and 48 WDs in the five GCs. The morphology of the observed HB stars in the five GCs was studied. The HB stars observed in far UV filters were found to be spread from blue-HB to extreme-HB regions whereas HB stars observed in near-UV filters were found in red-HB, blue-HB, and extreme-HB regions. The UVIT observed HB stars in NGC 4147 were found in the blue-HB region and belong to the second-generation population. There are two discrete groups among the HB stars of NGC 4147 visible in the BaF2−V versus BaF2 CMD. The empirical gap between the two groups was found to be 500 K with Teff range of 8,000 − 9,500 K for the first subgroup and 10,000 − 11,300 K for the second subgroup. The He abundances of HB stars in three GCs NGC 7492, NGC 5272, and NGC 6205 were estimated by matching the observed and theoretical HB stars, generated using BaSTI-IAC stellar models. The He abundances were found to be in the range of 0.247−0.350, 0.252−0.265, and 0.247−0.310 for NGC 7492, NGC 5272, and NGC 6205, respectively. There are 3 and 31 post-HB stars detected in NGC 5272 and NGC 6205, respectively. The UV-optical CMDs were able to detect only those post-HB stars which have evolved from HB mass (MHB) ≤ 0.55 M⊙. The UV-bright WD population of NGC 5272 and NGC 6205 were studied in this thesis and found that most of the hot WD candidates in NGC 6205 are extremely low mass (ELM) WDs which are evolved from the binary stellar systems in the cluster whereas the hot WD candidates of NGC 5272 are mostly lying on single stellar evolution WDs or high mass WDs cooling sequences. The cluster dynamics of GC NGC 4590 were studied using the normalized radial distribution of the observed BS and HB stars and found that the cluster belongs to Family II and is one of the youngest clusters among dynamically intermediate-age Galactic GCs. The dynamical age of the cluster is calculated using A + parameter and found to be 0.423 ± 0.096 Gyr.