Stellar Exotica in 47 Tucanae

Abstract

We present far-UV spectroscopy obtained with HST for 48 blue objects in the core of 47 Tuc. Based on their position in a FUV-optical colour-magnitude diagram, these were expected to include cataclysmic variables (CVs), blue stragglers (BSs), white dwarfs (WDs) and other exotic objects. For a subset of these sources, we also construct FUV-NIR SEDs. Based on our analysis of this extensive data set, we report the following main results. (1) We spectroscopically confirm 3 previously known or suspected CVs via the detection of emission lines and find new evidence for dwarf nova eruptions in two of these. (2) Only one other source in our spectroscopic sample exhibits marginal evidence for line emission, but predicted and observed CV numbers still agree to within a factor of about 2-3. (3) We have discovered a hot (T_eff = 8700 K), low-mass (M = 0.05 M_sun) secondary star in a previously known 0.8 day binary system. This exotic object is probably the remnant of a subgiant that has been stripped of its envelope and may represent the ``smoking gun'' of a recent dynamical encounter. (4) We have found a Helium WD, the second to be optically detected in 47 Tuc, and the first outside a millisecond-pulsar system. (5) We have discovered a BS-WD binary system, the first known in any globular cluster. (6) We have found two additional candidate WD binary systems with putative main sequence and subgiant companions. (7) We estimate the WD binary fraction in the core of 47 Tuc to be 15 +17/-9 (stat) +8/-7 (sys). (8) One BS in our sample may exceed twice the cluster turn-off mass, but the uncertainties are large. Taken as a whole, our study illustrates the wide range of stellar exotica that are lurking in the cores of GCs, most of which are likely to have undergone significant dynamical encounters. [abridged]

28 pages, 22 figures, 1 table, accepted for publication in ApJ; abstract below is abridged; new version corrects some typos and updates some references; a copy with some higher resolution figures is available from http://www.astro.soton.ac.uk/~christian (under "Research")

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• High Resolution X-ray Imaging of a Globular Cluster Core: Compact Binaries in 47Tuc
• A Deep Chandra Survey of the Globular Cluster 47 Tucanae: Catalog of Point Sources
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• Evolution and colours of helium-core white dwarf stars: the case of low metallicity progenitors
• Chandra Observation of the Globular Cluster NGC 6440 and the Nature of Cluster X-ray Luminosity Functions
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• A Chandra X-ray Study of the Globular Cluster M80
• Resolving the Controversy Over the Core Radius of 47 Tucanae (NGC 104)
• Faint Cataclysmic Variables in Quiescence: Globular Cluster and Field Surveys
• HST/FOS Eclipse Observations of the Nova-like Cataclysmic Variable UX Ursae Majoris
• Far-Ultraviolet Observations of the Globular Cluster NGC 2808 Revisited: Blue Stragglers, White Dwarfs and Cataclysmic Variables
• Unveiling the core of the Globular Cluster M15 in the Ultraviolet
• Time-Resolved Ultraviolet Spectroscopy of the SW Sex Star DW UMa: Confirmation of a Hidden White Dwarf and the UV Counterpart to Phase 0.5 Absorption Events
• Helium-Core White Dwarfs in Globular Clusters
• XMM-Newton X-ray and optical observations of the globular clusters M 55 and NGC 3201
• First Evidence of Circumstellar Disks around Blue Straggler Stars
• The X-Ray Binary X2127+119 in M15: Evidence for a Very-Low-Mass, Stripped-Giant Companion
• Serendipitous Discovery of a Cataclysmic Variable in the Globular Cluster NGC 6624
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• A Binary Origin for Blue Stragglers in Globular Clusters
• Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate
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• The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9
• The Na-O anticorrelation in horizontal branch stars. III. 47 Tuc and M 5
• An Analytic Technique for Constraining the Dynamical Origins of Multiple Star Systems Containing Merger Products
• Implications for the Formation of Blue Straggler Stars from HST Ultraviolet Observations of NGC 188
• Monte Carlo Simulations of Star Clusters - VII. The globular cluster 47 Tuc
• Discovery of a stripped red giant core in a bright eclipsing binary system
• WASP 1628+10 - an EL CVn-type binary with a very-low-mass stripped-red-giant star and multi-periodic pulsations
• Helium-Core White Dwarfs in the Globular Cluster NGC 6397
• New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae
• UVIT Open Cluster Study. I. Detection of a White Dwarf Companion to a Blue Straggler in M67: Evidence of Formation through Mass Transfer
• Biases on initial mass function determinations. III. Cluster masses derived from unresolved photometry
• Ejection of globular cluster interstellar media through ionization by white dwarfs
• Constraining Mass Transfer Histories of Blue Straggler Stars with COS Spectroscopy of White Dwarf Companions
• UltraCompCAT: a comprehensive Catalogue of Ultra-Compact and Short Orbital Period X-ray Binaries
• A Far-UV Survey of M 80: X-ray Source Counterparts, Strange Blue Stragglers and the Recovery of Nova T Sco
• UV Extinction Towards a Quiescent Molecular Cloud in the SMC
• 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)
• Spectral Energy Distribution of Blue Stragglers in the core of 47 Tucanae
• HST spectrum and timing of the ultra-compact X-ray binary candidate 47 Tuc X9
• UOCS.VI. UVIT/AstroSat detection of low-mass white dwarf companions to 4 more blue stragglers in M67
• A Measurement of Diffusion in 47 Tucanae
• Hα photometry of low mass stars in 47 Tucanae: chromospheric activity and exotica
• UVIT/ASTROSAT studies of Blue Straggler stars and post-mass transfer systems in star clusters: Detection of one more blue lurker in M67
• ALMA reveals sunburn: CO dissociation around AGB stars in the globular cluster 47 Tucanae
• Formation constraints indicate a black-hole accretor in 47 Tuc X9
• On the absence of symbiotic stars in globular clusters
• When do stars in 47 Tucanae lose their mass?
• Globular Clusters as Testbeds for Type Ia Supernovae
• Far-Ultraviolet Observation of the Globular Cluster NGC 6397
• A multi-wavelength survey of NGC\,6752: X-ray counterparts, two new dwarf novae, and a core-collapsed radial profile
• Globular Clusters UVIT Legacy Survey (GlobULeS) I. FUV-optical Color-Magnitude Diagrams for Eight Globular Clusters
• UV-Bright Stellar Populations and Their Evolutionary Implications in the Collapsed-Core Cluster M15
• Type Ia supernovae in globular clusters: observational upper limits
• UOCS-IX. AstroSat/UVIT study of the open cluster NGC 2818: Blue Stragglers, Yellow Stragglers, Planetary Nebula, and their membership
• Slow Decline and Rise of the Broad [OIII] Emission Line in Globular Cluster Black Hole Candidate RZ2109
• Constraining the LyC escape fraction from LEGUS star clusters with SIGNALS HII region observations: A pilot study of NGC 628
• Periodic X-ray sources in the Massive Globular Cluster 47 Tucanae: Evidence for Dynamically Formed Cataclysmic Variables
• Outliers: multicolour photometry guiding the search for evolved binary systems in the globular cluster 47 Tucanae
• Near-ultraviolet detections of four dwarf nova candidates in the globular cluster 47 Tucanae
• Detection of White Dwarf Companions to Blue Straggler Stars from UVIT Observations of M67
• Photometric variability of blue straggler stars in M67 with TESS and K2
• IIb or not IIb: A Catalog of ZTF Kilonova Imposters
• Stellar populations of the globular cluster NGC 5053 investigated using AstroSat-Ultra Violet Imaging Telescope
• An Extreme Ultra-Compact X-ray Binary in a Globular Cluster: Multi-Wavelength Observations of RZ2109 Explored in a Triple System Framework