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NGC 5139
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NGC 5139 (also known as Omega Centauri, GCL 24, ESO 270-SC11, and Caldwell 80) is a magnitude +3.9 globular cluster located 15,800 ±1.1 light-years away in the constellation Centaurus.
It is believed that omega Centauri is actually a remnant nuclei of a dwarf galaxy that was absorbed by the Milky Way.

In 150 A.D., Greco-Roman writer and astronomer Ptolemy catalogued this object in his Almagest as a star on the horse's back, "Quae est in principio scapulae". German lawyer and cartographer Johann Bayer used Ptolemy's data to designate this object "Omega Centauri" with his 1603 publication of Uranometria.
Using a telescope from the South Atlantic island of Saint Helena, English astronomer Edmond Halley rediscovered this object in 1677, listing it as a non-stellar object.

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Right ascension 13h 26m 47.28s, Declination -47° 28' 46.1"



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RE: Omega Centauri
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Title: The Kapteyn moving group is not tidal debris from omega Centauri
Author: Camila Navarrete, Julio Chanamé, Iván Ramírez, Andrés Meza, Guillem Anglada-Escudé, Evgenya Shkolnik

The Kapteyn moving group has been postulated as tidal debris from omega Centauri. If true, members of the group should show some of the chemical abundance patterns known for stars in the cluster. We present an optical and near-infrared high-resolution, high-S/N spectroscopic study of 14 stars of the Kapteyn group, plus 10 additional stars (the omega Cen-group) that, while not listed as members of the Kapteyn group as originally defined, have been nevertheless associated dynamically with omega Centauri. Abundances for Na, O, Mg, Al, Ca and Ba were derived from the optical spectra, while the strength of the chromospheric He I 10830 {\AA} line is studied as a possible helium abundance indicator. The resulting Na-O and Mg-Al patterns for stars of the combined Kapteyn and omega Cen-group samples do not resemble those of omega Centauri, and are not different from those of field stars of the Galactic halo. The distribution of equivalent widths of the He I 10830 {\AA} line is consistent with that found among non-active field stars. Therefore, no evidence is found for second-generation stars within our samples, which most likely rules out a globular-cluster origin. Moreover, no hint of the unique Ba-overabundance at the metal-rich end, well-established for omega Centauri stars, is seen among stars of the combined samples. Because this specific Ba pattern is present in omega Centauri irrespective of stellar generation, this would rule out the possibility that our entire sample might be composed of only first generation stars from the cluster. Finally, for the stars of the Kapteyn group, the possibility of an origin in the hypothetical omega Centauri's parent galaxy is disfavored by the different run of alpha-elements with metallicity between our targets and stars from present-day dwarf galaxies.

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Title: HST/ACS Imaging of Omega Centauri: Optical Counterparts of Chandra X-Ray Sources
Authors: Adrienne M. Cool (SFSU), Daryl Haggard (NU/CIERA), Tersi Arias (SFSU/UCLA), Michelle Brochmann (SFSU/UW), Jason Dorfman (SFSU/Bays Mountain Park Obs.), April Gafford (SFSU/JATO Aviation), Vivian White (SFSU/ASP), Jay Anderson (STScI)

We present results of a search for optical counterparts of X-ray sources in and toward the globular cluster Omega Centauri (NGC 5139) using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The ACS data consist of a mosaic of Wide Field Channel (WFC) images obtained using F625W, F435W, and F658N filters; with 9 pointings we cover the central ~10'x10' of the cluster and encompass 109 known Chandra sources. We find promising optical counterparts for 59 of the sources, ~40 of which are likely to be associated with the cluster. These include 27 candidate cataclysmic variables (CVs), 24 of which are reported here for the first time. Fourteen of the CV candidates are very faint, with absolute magnitudes in the range M_625 = 10.4 - 12.6, making them comparable in brightness to field CVs near the period minimum discovered in the SDSS (Gansicke et al. 2009). Additional optical counterparts include three BY Dra candidates, a possible blue straggler, and a previously-reported quiescent low-mass X-ray binary (Haggard et al. 2004). We also identify three foreground stars and 11 probable active galactic nuclei. Finally, we report the discovery of a group of seven stars whose X-ray properties are suggestive of magnetically active binaries, and whose optical counterparts lie on or very near the metal-rich anomalous giant and subgiant branches in {\omega} Cen. If the apparent association between these seven stars and the RGB/SGB-a stars is real, then the frequency of X-ray sources in this metal-rich population is enhanced by a factor of at least five relative to the other giant and subgiant populations in the cluster. If these stars are not members of the metal-rich population, then they bring to 20 the total number of red stragglers (also known as sub-subgiants) that have been identified in {\omega} Cen, the largest number yet known in any globular cluster.

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Title: The dynamics of the outer parts of omega Centauri
Authors: G. S. Da Costa

The multi-object fibre-fed spectrograph AAOmega at the Anglo-Australian Telescope has been used to establish and measure accurate (<1 kms-1) radial velocities for a new sample of members in the outer parts of the stellar system omega Centauri. The new sample more than doubles the number of known members with precise velocities that lie between 25' and 45' from the cluster center. Combining this sample with earlier work confirms that the line-of-sight velocity dispersion of omega Cen remains approximately constant at ~6.5 kms-1 in the outer parts of the cluster, which contain only a small fraction of the total cluster stellar mass. It is argued that the approximately constant velocity dispersion in the outer regions is most likely a consequence of external influences, such as the tidal shock heating that occurs each time omega Cen crosses the Galactic plane. There is therefore no requirement to invoke dark matter or non-standard gravitational theories.

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Space Telescope Science Institute astronomers Jay Anderson and Roeland van der Marel discuss their in-depth study of the giant cluster Omega Centauri. The team used Hubble's exquisite resolving power to measure the positions for stars in 2002 and 2006. From these measurements, they can predict the stars' future movement.

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The globular star cluster Omega Centauri has caught the attention of sky watchers ever since the ancient astronomer Ptolemy first catalogued it 2,000 years ago. Ptolemy, however, thought Omega Centauri was a single star. He didn't know that the "star" was actually a beehive swarm of nearly 10 million stars, all orbiting a common center of gravity. The stars are so tightly crammed together that astronomers had to wait for the powerful vision of NASA's Hubble Space Telescope to peer deep into the core of the "beehive" and resolve individual stars. Hubble's vision is so sharp it can even measure the motion of many of these stars, and over a relatively short span of time.
Analysing archived images taken over a four-year period by Hubble's Advanced Camera for Surveys, astronomers have made the most accurate measurements yet of the motions of more than 100,000 cluster inhabitants, the largest survey to date to study the movement of stars in any cluster. A precise measurement of star motions in giant clusters can yield insights into how stellar groupings formed in the early universe, and whether an "intermediate mass" black hole, one roughly 10,000 times as massive as our Sun, might be lurking among the stars.

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Title: VLT Kinematics for omega Centauri: Further Support for a Central Black Hole
Authors: Eva Noyola, Karl Gebhardt, Markus Kissler-Patig, Nora Lutzgendorf, Behrang Jalali, P. Tim de Zeeuw, Holger Baumgardt

The Galactic globular cluster omega Centauri is a prime candidate for hosting an intermediate mass black hole. Recent measurements lead to contradictory conclusions on this issue. We use VLT-FLAMES to obtain new integrated spectra for the central region of omega Centauri. We combine these data with existing measurements of the radial velocity dispersion profile taking into account a new derived center from kinematics and two different centers from the literature. The data support previous measurements performed for a smaller field of view and show a discrepancy with the results from a large proper motion data set. We see a rise in the radial velocity dispersion in the central region to 22.8±1.2 km/s, which provides a strong sign for a central black hole. Isotropic dynamical models for omega Centauri imply black hole masses ranging from 3.0 to 5.2x10^4 solar masses depending on the center. The best-fitted mass is 4.7±1.0x10^4 solar masses.

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Title: Giants in the globular cluster omega Centauri: dust production, mass loss and distance
Authors: Iain McDonald, Jacco Th. van Loon, Leen Decin, Martha L. Boyer, Andrea K. Dupree, Aneurin Evans, Robert D. Gehrz, Charles E. Woodward

We present spectral energy distribution modelling of 6875 stars in omega Centauri, obtaining stellar luminosities and temperatures by fitting literature photometry to state-of-the-art MARCS stellar models. By comparison to four different sets of isochrones, we provide a new distance estimate to the cluster of 4850 ±200 (random) ±120 (systematic error) pc, a reddening of E(B-V) = 0.08 ±0.02 ±0.02 mag and a differential reddening of Delta[E(B-V)] < 0.02 mag for an age of 12 Gyr. Several new post-early-AGB candidates are also found. Infra-red excesses of stars were used to measure total mass-loss rates for individual stars down to ~7 x 10^-8 Msun/yr. We find a total dust mass-loss rate from the cluster of 1.3 (+0.8/-0.5) x 10^-9 Msun/yr, with the total gas mass-loss rate being > 1.2 (+0.6/-0.5) x 10^-6 Msun/yr. Half of the cluster's dust production and 30% of its gas production comes from the two most extreme stars - V6 and V42 - for which we present new Gemini/T-ReCS mid-infrared spectroscopy, possibly showing that V42 has carbon-rich dust. The cluster's dust temperatures are found to be typically >~550 K. Mass loss apparently does not vary significantly with metallicity within the cluster, but shows some correlation with barium enhancement, which appears to occur in cooler stars, and especially on the anomalous RGB. Limits to outflow velocities, dust-to-gas ratios for the dusty objects and the possibility of short-timescale mass-loss variability are also discussed in the context of mass loss from low-metallicity stars. The ubiquity of dust around stars near the RGB-tip suggests significant dusty mass loss on the RGB; we estimate that typically 0.20--0.25 Msun of mass loss occurs on the RGB. From observational limits on intra-cluster material, we suggest the dust is being cleared on a timescale of <~10^5 years.

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Omega Centauri is one of the finest jewels of the southern hemisphere night sky, as ESO's latest stunning image beautifully illustrates. Containing millions of stars, this globular cluster is located roughly 17 000 light-years from Earth in the constellation of Centaurus.
Sparkling away at magnitude 3.7 and appearing nearly as large as the full moon on the southern night sky, Omega Centauri is visible with the unaided eye from a clear, dark observing site. Even through a modest amateur telescope, the cluster is revealed as an incredible, densely packed sphere of glittering stars. But astronomers need to use the full power of professional telescopes to uncover the amazing secrets of this beautiful globular cluster.
This new image is based on data collected with the Wide Field Imager (WFI), mounted on the 2.2-metre diameter Max-Planck/ESO telescope, located at ESO's La Silla observatory, high up in the arid mountains of the southern Atacama Desert in Chile. Omega Centauri is about 150 light-years across and is the most massive of all the Milky Way's globular clusters. It is thought to contain some ten million stars!

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