Globular Cluster UVIT Legacy Survey (GlobULeS) – II. Evolutionary status of hot stars in M3 and M13

We present the first comprehensive study of the most massive globular cluster, Omega Centauri, in the far-ultraviolet (FUV), extending from the center to ∼28% of the tidal radius using the Ultraviolet Imaging Telescope on board AstroSat. A comparison of the FUV-optical color–magnitude diagrams with available canonical models reveals that horizontal branch (HB) stars bluer than the knee (hHBs) and the white dwarfs (WDs) are fainter in the FUV by ∼0.5 mag than model predictions. They are also fainter than their counterparts in M13, another massive cluster. We simulated HB with at least five subpopulations, including three He-rich populations with a substantial He enrichment of Y up to 0.43 dex, to reproduce the observed FUV distribution. We find the He-rich younger subpopulations to be radially more segregated than the He-normal older ones, suggesting an in situ enrichment from older generations. The ω Cen hHBs span the same Teff range as their M13 counterparts, but some have smaller radii and lower luminosities. This may suggest that a fraction of ω Cen hHBs are less massive than those of M13, similar to the result derived from earlier spectroscopic studies of outer extreme HB stars. The WDs in ω Cen and M13 have similar luminosity–radius–Teff parameters, and 0.44–0.46 M⊙ He-core WD model tracks evolving from progenitors with Y = 0.4 dex are found to fit the majority of these. This study provides constraints on the formation models of ω Cen based on the estimated range in age, [Fe/H], and Y (in particular) for the HB stars

GlobULeS. IV. UVIT/AstroSat detection of extremely low mass white dwarf companions to blue straggler stars in NGC 362

We report the discovery of extremely low mass white dwarfs (ELM WDs) as a companion of blue straggler stars (BSSs) in the Galactic globular cluster NGC 362 using images from AstroSat's Ultra Violet Imaging Telescope (UVIT). Spectral energy distributions (SEDs) for 26 far-UV (FUV) bright member BSSs are created using data from the UVIT, the UltraViolet and Optical Telescope (UVOT), Gaia EDR3, and the 2.2 m ESO/MPI telescope. A single SED is fitted to 14 BSSs, whereas double-SED fits revealed ELM WDs as binary companions in 12 of the 26 BSSs studied. The effective temperature, radius, luminosity, and mass of the 12 ELM WDs are found to have a range of Teff = 9750−18,000 K, R = 0.1−0.4 R⊙, L = 0.4−3.3 L⊙, and M = 0.16−0.20 M⊙. These suggest that 12 BSSs are post-mass-transfer systems formed through the case A/B mass transfer pathway. To the best of our knowledge, this is the first finding of ELM WDs as companions to BSSs in globular clusters. This cluster is known to have a binary BSS sequence, and the 12 binary and 14 single BSSs (as classified by the SEDs) follow the mass transfer and collisional sequence of BSSs in the color–magnitude diagram. The cooling ages of nine of the ELM WDs are found to be younger than 500 Myr. Though the binary BSSs may have formed during the core collapse (∼200 Myr) or as part of the dynamical evolution of the cluster, they provide new insights on the dynamics of this cluster