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Post Info TOPIC: NGC 6752


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Posts: 130044
Date:
ESO 141-SC30
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Title: Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6752
Author: Phyllis M. Lugger, Haldan N. Cohn, Adrienne M. Cool, Craig O. Heinke, Jay Anderson

We have searched for optical identifications for 39 Chandra X-ray sources that lie within the 1.9 arcmin half-mass radius of the nearby (d = 4.0 kpc), core-collapsed globular cluster, NGC 6752, using deep Hubble Space Telescope ACS/WFC imaging in B435, R625, and H alpha. Photometry of these images allows us to classify candidate counterparts based primarily on color-magnitude and colour-colour diagram location. The color-color diagram is particularly useful for quantifying the H alpha line equivalent width. In addition to recovering 11 previously detected optical counterparts, we propose 20 new optical IDs. In total, there are 16 likely or less certain cataclysmic variables (CVs), nine likely or less certain chromospherically active binaries, three galaxies, and three active galactic nuclei (AGNs). The latter three sources, which had been identified as likely CVs by previous investigations, now appear to be extragalactic objects based on their proper motions. As we previously found for NGC 6397, the CV candidates in NGC 6752 fall into a bright group that is centrally concentrated relative to the turnoff-mass stars and a faint group that has a spatial distribution that is more similar to that of the turnoff-mass stars. This is consistent with an evolutionary scenario in which CVs are produced by dynamical interactions near the cluster center and diffuse to larger radius orbits as they age.

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Posts: 130044
Date:
GCL 108
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NGC 6752 (also ESO 141-SC30 and GCL 108) is a magnitude +5.4 globular star cluster located 13000 light-years away in the constellation Pavo.

The cluster was discovered by Scottish astronomer James Dunlop using a homemade 9-foot 22.86 cm (9 inch) f/12 speculum Newtonian reflector at Paramatta, New South Wales, Australia, on the 30th June 1826.

Right Ascension 19h 10m 52.11s, Declination -59° 59' 04.4"

It lies 1.5 degrees east of 5th magnitude Omega Pavonis. The nearest bright star is Alpha Pavonis, which lies 3.25 degrees north and 9.25 degrees east
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Posts: 130044
Date:
RE: NGC 6752
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Title: A multi-wavelength survey of NGC 6752: X-ray counterparts, two new dwarf novae, and a core-collapsed radial profile
Authors: G. S. Thomson, C. Knigge, A. Dieball, T. J. Maccarone, A. Dolphin, D. Zurek, K. S. Long, M. Shara, A. Sarajedini

We present the results of a multi-wavelength (FUV to I-band) survey of the stellar populations of the globular cluster NGC 6752, using STIS, ACS and WFC3 on board the Hubble Space Telescope. We have confirmed that two previously identified CV candidates are, in fact, dwarf novae which underwent outbursts during our observations. We have also identified previously unknown optical counterparts to two X-ray sources. We estimate the position of the centre of the cluster, and show that the stellar density profile is not well described by a single King model, indicating that this cluster is in a core-collapsed or post-core collapse phase. The colour-magnitude diagram shows a well-populated horizontal branch, numerous blue stragglers and white dwarfs (WDs), as well as 87 sources in the gap region where we expect to find WD - main sequence binaries, including cataclysmic variables (CVs). The X-ray sources and WD binary systems are the most centrally concentrated populations, with dynamically estimated characteristic masses >1.1 solar masses and >0.8 solar masses, respectively.

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Title: Chemical tagging of three distinct populations of red giants in the globular cluster NGC 6752
Authors: Eugenio Carretta (1), Angela Bragaglia (1), Raffaele Gratton (2), Sara Lucatello (2), Valentina D'Orazi (3) ((1) INAF-Osservatorio Astronomico di Bologna, (2) INAF-Osservatorio Astronomico di Padova, (3) Dept. of Physics and Astronomy, Macquarie University)

We present aluminium, magnesium, and silicon abundances in the metal-poor globular cluster NGC 6752 for a sample of more than 130 red giants with homogeneous oxygen and sodium abundances. We find that [Al/Fe] shows a spread of about 1.4 dex among giants in NGC 6752 and is anticorrelated with [Mg/Fe] and [O/Fe] and correlated with [Na/Fe] and [Si/Fe]. These relations are not continuous in nature, but the distribution of stars is clearly clustered around three distinct Al values, low, intermediate, and high. These three groups nicely correspond to the three distinct sequences previously detected using Stromgren photometry along the red giant branch. These two independent findings strongly indicate the existence of three distinct stellar populations in NGC 6752. Comparing the abundances of O and Mg, we find that the population with intermediate chemical abundances cannot originate from material with the same composition of the most O- and Mg-poor population, diluted by material with that of the most O- and Mg-rich one. This calls for different polluters.

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Posts: 130044
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Title: Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects
Authors: V. Dobrovolskas (1), A. Kucinskas (2,1), S. M. Andrievsky (3,4), S. A. Korotin (3), T. V. Mishenina (3), P. Bonifacio (4), H.-G. Ludwig (5), E. Caffau (5,4) ((1) Vilnius Univ. Astronomical Observatory, (2) Institute of Theoretical Physics and Astronomy, Vilnius Univ., (3) Department of Astronomy and Astronomical Observatory, Odessa National University, (4) GEPI, Obs. de Paris, (5) Zentrum für Astronomie der Universität Heidelberg)

Aims: We study the effects related to departures from non-local thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red giant stars in Galactic globular cluster NGC 6752, to assess their influence on the formation of Ba II lines.
Methods: One-dimensional (1D) local thermodynamic equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D ATLAS stellar model atmospheres. The three-dimensional (3D) LTE abundances were obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE abundances using 3D-1D abundance corrections, i.e., the differences between the abundances obtained from the same spectral line using the 3D hydrodynamical (CO5BOLD) and classical 1D (LHD) stellar model atmospheres.
Results: The mean 1D barium-to-iron abundance ratios derived for 20 giants are _{1D NLTE} = 0.05 ±0.06 (stat.) ±0.08 (sys.). The 3D-1D abundance correction obtained for 8 giants is small (~+0.05 dex), thus leads to only minor adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for the 20 giants, _{3D+NLTE} = 0.10 ±0.06(stat.) ±0.10(sys.). The intrinsic abundance spread between the individual cluster stars is small and can be explained in terms of uncertainties in the abundance determinations.
Conclusions: Deviations from LTE play an important role in the formation of barium lines in the atmospheres of red giants studied here. The role of 3D hydrodynamical effects should not be dismissed either, even if the obtained 3D-1D abundance corrections are small. This result is a consequence of subtle fine-tuning of individual contributions from horizontal temperature fluctuations and differences between the average temperature profiles in the 3D and 1D model atmospheres: owing to the comparable size and opposite sign, their contributions nearly cancel each other.

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Posts: 130044
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Title: Nitrogen abundances in giant stars of the globular cluster NGC 6752
Authors: David Yong (1), Frank Grundahl (2), Jennifer A. Johnson (3), Martin Asplund (4) ((1) RSAA, Mt Stromlo Observatory, (2) Univ. of Aarhus, (3) Ohio State University, (4) MPA Garching)

We present N abundances for 21 bright giants in the globular cluster NGC 6752 based on high-resolution UVES spectra of the 3360A NH lines. We confirm that the Stromgren c1 index traces the N abundance and find that the star-to-star N abundance variation is 1.95 dex, at the sample's luminosity. We find statistically significant correlations, but small amplitude variations, between the abundances of N and alpha-, Fe-peak, and s-process elements. Analyses using model atmospheres with appropriate N, O, Na, and Al abundances would strengthen, rather than mute, these correlations. If the small variations of heavy elements are real, then the synthesis of the N anomalies must take place in stars which also synthesize alpha-, Fe-peak, and s-process elements. These correlations offer support for contributions from both AGB and massive stars to the globular cluster abundance anomalies.

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