Investigating the ultra-violet bright stars in star clusters

Abstract

Star clusters are gravitationally bound systems composed of a large number of stars, ranging from thousands to millions. As the dynamic age of the cluster is less than the age of the universe, dynamical interactions happening among stars within clusters result in stellar exotica such as blue straggler stars (BSSs), cataclysmic variables, etc. Globular clusters (GCs), being old and dense, host low-mass stars in various stages of evolution, making them ideal for investigating the end stages of low-mass star evolution and the origins of exotic stellar populations, which are not yet fully understood. Ultraviolet (UV) studies conducted so far using HST and GALEX data have revealed the presence of intriguing stars such as extreme horizontal branch (EHB) stars and blue hook (BHk) stars in GCs, making them enigmatic systems. Furthermore, the discovery of multiple stellar populations (MSPs) in both types of clusters through UV observations has provided new perspectives on their formation and evolution. In this thesis, we examined the properties of late-stage, UV-bright stellar populations and the exotic BS and yellow straggler (YS) stars in targeted star clusters. We primarily have employed the UV data from the Ultraviolet Imaging Telescope (UVIT) onboard AstroSat, India’s first multi-wavelength observatory in both far-UV (FUV) and near-UV (NUV) bands. We have conducted an in-depth analysis of four clusters, consisting of two intermediate-mass GCs (NGC 1261 and NGC 2298) and two open clusters (OCs) (NGC 188 and NGC 2818). We performed the PSF photometry on the UV images obtained from UVIT and further combined the UVIT-detected sources with the other available archival data. In the case of GCs, we utilised the HST data to find the optical counterparts of the UV detected sources for the central region (∼2.07 square) of the cluster, and Gaia DR2 and EDR3 catalogue for the outer region of the cluster not covered with HST. To obtain the proper-motion (PM) members of the clusters, we used Gaia DR2 and EDR3 catalogues for all the clusters. In the case of GCs, NGC 1261 and NGC 2298, we explored their horizontal branch (HB) morphology. In NGC 1261, we constructed UV colour-magnitude diagrams (CMDs) in combination with HST, Gaia DR2, and ground-based optical photometry for member stars. We detected the full HB in NUV, and blue HB (BHB) in the FUV and identified two EHB stars. HB stars have a tight sequence in UV-optical CMDs well fitted with isochrones generated (12.6 Gyr age, [Fe/H] = −1.27 dex metallicity) using updated BaSTI-IAC models. Effective temperatures (Tef f ), luminosities and radii of bright HB stars were estimated using spectral energy distribution (SED). As we detect the complete sample of UV bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump (Tef f ∼11,500 K). The two EHB stars, fitted well with single spectra, have Tef f = 31,000 K and a mass = 0.495 M and follow the same Tef f−Radius relation of the BHB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (either due to rotation or enhanced Helium) OR early hot-flash scenario. In NGC 2298, UV-optical CMDs are constructed for member stars in combination with HST UV Globular Cluster Survey (HUGS) data for the central region and Gaia EDR3 and ground-based photometric data for the outer region. BHB sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs. They are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated Tef f , radius, and luminosity of HB stars, using best SED fits, were compared with various HB models. BHB stars span a temperature range from 7,500-12,250 K. The three hot HB stars have 35,000-40,000 K. In contrast; one star has around ∼100,000 K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of EHB stars formed through an early hot-flasher scenario; one is likely to be an EHB star with probable helium enrichment, the hottest HB star is about to enter the white dwarf (WD) cooling phase, could have evolved from BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution. In the old OC NGC 188, UVIT data is utilised in combination with optical photometric data to construct the optical and UV CMDs. In the FUV images, we detect only hot and bright BSSs, one hot subdwarf, and one WD candidate. In the NUV images, we detect members up to a faintness limit of ∼22 mag, including 21 BSSs, 2 YSSs, and one WD candidate. This study presents the first NUV-optical CMDs, which are overlaid with updated BaSTI-IAC isochrones and WD cooling sequences, which are found to fit well with the observed CMDs. We use SED fitting to estimate the Tef f radii, and luminosities of the UV-bright stars. We find the cluster to have an HB population with three stars (Tef f = 4,750−21,000 K). We also detect two YSSs, with one of them with UV excess connected to its binarity and X-ray emission. In the intermediate-age OC NGC 2818 that has a Planetary nebula (PN) within the field, we present the first FUV imaging results. We explore whether the PN is a member of the cluster using images taken from the UVIT. We detect four bright and hot BSSs and two YSSs based on their location in the optical and FUV-optical CMDs. The theoretical isochrones more or less fit the observed distribution of detected stars in all the CMDs. Based on the parameters estimated using SED, we infer that the BSSs are either collisional products or might have undetectable WD companions. Our photometric analysis of YSSs confirms their binarity, consistent with the spectroscopic results. We find the YSSs to be formed through a mass-transfer scenario and the hot components are likely to be A-type subdwarfs. A comparison of the radial velocity (RV), Gaia EDR3 PM of the PN with the cluster members, and reddening towards the PN and the cluster does not rule out the membership of the PN. Using SED, the estimated stellar parameters of the PN’s central star match well with the previous estimations. Comparing the central star’s position with theoretical post-AGB (pAGB) models suggest that it has already entered the WD cooling phase, and its mass is deduced to be ∼0.66 M . The corresponding progenitor mass turns out to be ∼2.1 M , comparable to the turn-off mass of the cluster, implying that the progenitor could have formed in the cluster. We suggest that the NGC 2818 might be one of the few known clusters to host a PN, providing a unique opportunity to test stellar evolution models. Overall, our study showcases the effectiveness of UVIT in resolving stars in GCs, such as NGC 1261 and NGC 2298 in FUV, and identifying the hot companions to BSSs and YSSs in OCs, providing more insight into their formation pathways. Additionally, its multiple filters facilitate in constructing the multi-wavelength SEDs and estimating parameters of the hot stellar populations.