Browsing by Author "Shara, Michael"

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    Detection of a White Dwarf Companion to a Blue Straggler Star in the Outskirts of Globular Cluster NGC 5466 with the Ultraviolet Imaging Telescope (UVIT)
    (2019) Sahu, Snehalata; Subramaniam, Annapurni; Simunovic, Mirko; Postma, J.; Cote, Patrick; Rao, N. Kameswera; Geller, Aaron M.; Leigh, Nathan; Shara, Michael; Puzia, Thomas H.; Stetson, Peter B.
    We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from four filters of UVIT, with GALEX, GALA, and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T-eff = 8000(-250)(+1000) K, R/R-circle dot = 1.44 +/- 0.05) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T-eff = 32,000 +/- 2000 K, R/R-circle dot = 0.021 +/- 0.007) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GALA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low-density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low-density environments.
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    Globular Cluster UVIT Legacy Survey (GlobULeS) - I. FUV-optical colour-magnitude diagrams for eight globular clusters
    (2022) Sahu, Snehalata; Subramaniam, Annapurni; Singh, Gaurav; Yadav, Ramakant; Valcarce, Aldo R.; Choudhury, Samyaday; Rani, Sharmila; Prabhu, Deepthi S.; Chung, Chul; Cote, Patrick; Leigh, Nathan; Geller, Aaron M.; Chatterjee, Sourav; Rao, N. Kameswara; Bandyopadhyay, Avrajit; Shara, Michael; Dalessandro, Emanuele; Pandey, Gajendra; Postma, Joesph E.; Hutchings, John; Simunovic, Mirko; Stetson, Peter B.; Thirupathi, Sivarani; Puzia, Thomas; Sohn, Young-Jong
    We present the first results of eight globular clusters (GCs) from the AstroSat/UVIT Legacy Survey programme GlobULeS based on the observations carried out in two far-ultraviolet (FUV) filters (F148W and F169M). The FUV-optical and FUV-FUV colour-magnitude diagrams (CMDs) of GCs with the proper motion membership were constructed by combining the Ultra-Violet Imaging Telescope (UVIT) data with Hubble Space Telescope (HST) UV Globular Cluster Survey data for inner regions and Gaia Early Data Release for regions outside the HST's field. We detect sources as faint as F148W similar to 23.5 mag, which are classified based on their locations in CMDs by overlaying stellar evolutionary models. The CMDs of eight GCs are combined with the previous UVIT studies of three GCs to create stacked FUV-optical CMDs to highlight the features/peculiarities found in the different evolutionary sequences. The FUV (F148W) detected stellar populations of 11 GCs comprise 2816 horizontal branch (HB) stars [190 extreme HB (EHB) candidates], 46 post-HB (pHB), 221 blue straggler stars (BSSs), and 107 white dwarf (WD) candidates. We note that the blue HB colour extension obtained from F148W - G colour and the number of FUV detected EHB candidates are strongly correlated with the maximum internal helium (He) variation within each GC, suggesting that the FUV-optical plane is the most sensitive to He abundance variations in the HB. We discuss the potential science cases that will be addressed using these catalogues including HB morphologies, BSSs, pHB, and WD stars.
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    Globular Cluster UVIT Legacy Survey (GlobULeS). III. Omega Centauri in Far-ultraviolet
    (2022) Prabhu, Deepthi S.; Subramaniam, Annapurni; Sahu, Snehalata; Chung, Chul; Leigh, Nathan W. C.; Dalessandro, Emanuele; Chatterjee, Sourav; Rao, N. Kameswara; Shara, Michael; Cote, Patrick; Choudhury, Samyaday; Pandey, Gajendra; Valcarce, Aldo A. R.; Singh, Gaurav; Postma, Joesph E.; Rani, Sharmila; Bandyopadhyay, Avrajit; Geller, Aaron M.; Hutchings, John; Puzia, Thomas; Simunovic, Mirko; Sohn, Young-Jong; Thirupathi, Sivarani; Yadav, Ramakant Singh
    We present the first comprehensive study of the most massive globular cluster, Omega Centauri, in the far-ultraviolet (FUV), extending from the center to similar 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 similar to 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 omega Cen hHBs span the same T (eff) range as their M13 counterparts, but some have smaller radii and lower luminosities. This may suggest that a fraction of omega 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 omega Cen and M13 have similar luminosity-radius-T (eff) parameters, and 0.44-0.46 M (circle dot) 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 omega Cen based on the estimated range in age, [Fe/H], and Y (in particular) for the HB stars.