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NGC 6397
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Title: Far-Ultraviolet Observation of the Globular Cluster NGC 6397
Author: Andrea Dieball, Armin Rasekh, Christian Knigge, Michael Shara, David Zurek

We present an observational far-UV (FUV) and near-UV (NUV) study of the core region of the globular cluster NGC 6397. The observations were obtained with the Space Telescope Imaging Spectrograph (STIS, FUV), and the Wide Field Camera 3 (WFC3, NUV) on board the Hubble Space Telescope. Here, we focus on the UV bright stellar populations such as blue stragglers (BSs), white dwarfs (WDs) and cataclysmic variables (CVs). We present the first FUV-NUV color-magnitude diagram (CMD) for this cluster. To support our classification of the stellar populations, we compare our FUV-NUV CMD with optical data from the ACS Survey of Galactic Globular Clusters. The FUV-NUV CMD indicates 16 sources located in the WD area, and ten BSs within the 25"x 25" of the STIS FUV data. Eighteen Chandra X-ray sources are located within the FUV field of view. Thirteen of those have a NUV counterpart, of which nine sources also have a FUV counterpart. Out of those, five sources are previously suggested CVs, and indeed all five are located in the WD/CV region in our FUV-NUV CMD. Another CV only has a FUV but no NUV counterpart. We also detect a NUV (but no FUV) counterpart to the MSP located in the core of this cluster. The NUV lightcurves of the CVs and MSP show flickering behaviour typical of CVs. We found that the BSs and CVs are the most centrally concentrated population. This might be an effect of mass segregation or indicate the preferred birth place of BSs and CVs via dynamical interactions in the dense core region of GCs. HB stars are the least centrally concentrated population and absent in the innermost area of the core.

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Posts: 131433
Date:
Caldwell 86
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NGC 6397 (also Caldwell 86, ESO 181-SC4 and GCl 74) is a magnitude +6.68 globular star cluster located 7,200 light-years away in the constellation Ara.

The cluster was discovered by French astronomer Nicolas Louis de Lacaille using a 1.27 cm (0.5 inch), 8x magnification, refractor at the Cape of Good Hope on the 18th July 1752.

Right Ascension  RA 17h 40m 42.20s | Dec -53° 40' 23.70"

The cluster contains around 400,000 stars, and can be seen with the naked eye under good observing conditions
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gclust_w11.jpg

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NGC 6397
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Title: Deep HST Imaging in NGC 6397: Stellar Dynamics
Authors: J. S. Heyl, H. Richer, J. Anderson, G. Fahlman, A. Dotter, J. Hurley, J. Kalirai, R. M. Rich, M. Shara, P. Stetson, K. H. Woodley, D. Zureko

Multi-epoch observations with ACS on HST provide a unique and comprehensive probe of stellar dynamics within NGC 6397. We are able to confront analytic models of the globular cluster with the observed stellar proper motions. The measured proper motions probe well along the main sequence from 0.8 to below 0.1 solar masses as well as white dwarfs younger than one gigayear. The observed field lies just beyond the half-light radius where standard models of globular cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution) make very robust predictions for the stellar proper motions as a function of mass. The observed proper motions show no evidence for anisotropy in the velocity distribution; furthermore, the observations agree in detail with a straightforward model of the stellar distribution function. We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results. Using the observed proper motions of the main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397 of 2.2^{+0.5}_{-0.7} kpc and a mass of the cluster of 1.1 ± 0.1 x 10^5 solar masses at the photometric distance of 2.53 kpc. One of the main-sequence stars appears to travel on a trajectory that will escape the cluster, yielding an estimate of the evaporation timescale, over which the number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars, providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr.

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Posts: 131433
Date:
2MASS J17410651-5343290
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Title: A super-Li rich turnoff star in NGC 6397 - the puzzle persists 
Authors: Andreas Koch, Karin Lind, Ian B. Thompson, R. Michael Rich 

This presentation focuses on a recently discovered super-Li rich turnoff star in the old, metal poor globular cluster NGC 6397 (Koch et al. 2011, ApJL, 738, L29). Its unusually high NLTE lithium abundance of A(7Li) = 4.21, the highest Li enhancement found in a Galactic GC dwarf star to date, has defied any unambiguous explanation through canonical enrichment channels. Spectra of the star show no convincing evidence for binarity, and measured line strengths and chemical element abundance ratios are fully compatible with other turnoff stars in this GC, seemingly ruling out mass transfer from an AGB companion as origin of the high A(Li). A possible cause is an interaction with a red giant that has undergone cool bottom processing. 

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Posts: 131433
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NGC 6397
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Title: 1Gyr in the Life of the Globular Cluster NGC 6397
Authors: Douglas C. Heggie, Mirek Giersz

M4 and NGC 6397 are two very similar galactic globular clusters, which differ mainly in their surface brightness profile. M4 has a classic King-like profile, whereas NGC 6397 has a more concentrated profile, which is often interpreted as that of a post-core collapse cluster. Heggie & Giersz (2008), however, found that M4 is also a post-core collapse cluster, and Giersz & Heggie (2009) concluded that the main reason for the difference between the two surface brightness profiles is fluctuations. This conclusion was reached on the basis of Monte Carlo models, however, and in the present Letter we verify that similar fluctuations occur in N-body models. The models were initialised by generating initial conditions from the Monte Carlo model of NGC 6397 at the simulated age of 12Gyr, and one was followed for 1Gyr. The new models help to clarify the nature of the fluctuations, which have the nature of semi-regular oscillations with a time scale of order 10^8yr. They are influenced by the dynamical role which is played by primordial binaries in the evolution of the core.

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Title: A Deep HST Study of the Globular Cluster NGC 6397: Reduction Methods
Authors: Jay Anderson, Ivan R. King, Harvey B. Richer, Gregory G. Fahlman, Brad M. S. Hansen, Jarrod Hurley, Jasonjot S. Kalirai, R. Michael Rich, Peter B. Stetson

We describe here the reduction methods that we developed to study the faintest red dwarfs and white dwarfs in an outer field of NGC6397, which was observed by HST for 126 orbits in 2005. The particular challenge of this data set is that the faintest stars are not readily visible in individual exposures, so special care must be taken to combine the information in all the exposures in order to identify and measure them. Unfortunately, it is hard to find the faintest stars without also finding a large number of faint galaxies, so we developed specialized tools to distinguish between the point-like stars and the barely resolved galaxies. We found that artificial-star tests, while obviously necessary for completeness determination, can also play an important role in helping us optimise our finding and measuring algorithms. Although this paper focuses on this data set specifically, many of the techniques are new and might find application in other work, particularly when a large number of images is available for a single field.

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Posts: 131433
Date:
NGC6397
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Title: Deep ACS Imaging in the Globular Cluster NGC6397: Dynamical Models
Authors: Jarrod R. Hurley, Michael M. Shara, Harvey B. Richer, Ivan R. King, D. Saul Davis, Jason S. Kalirai, Brad M. S. Hansen, Aaron Dotter, Jay Anderson, Gregory G. Fahlman, R. Michael Rich

We present N-body models to complement deep imaging of the metal-poor core-collapsed cluster NGC6397 obtained with the Hubble Space Telescope. All simulations include stellar and binary evolution in-step with the stellar dynamics and account for the tidal field of the Galaxy. We focus on the results of a simulation that began with 100000 objects (stars and binaries), 5% primordial binaries and Population II metallicity. After 16 Gyr of evolution the model cluster has about 20% of the stars remaining and has reached core-collapse. We compare the colour-magnitude diagrams of the model at this age for the central region and an outer region corresponding to the observed field of NGC6397 (about 2-3 half-light radii from the cluster centre). This demonstrates that the white dwarf population in the outer region has suffered little modification from dynamical processes - contamination of the luminosity function by binaries and white dwarfs with non-standard evolution histories is minimal and should not significantly affect measurement of the cluster age. We also show that the binary fraction of main-sequence stars observed in the NGC6397 field can be taken as representative of the primordial binary fraction of the cluster. For the mass function of the main-sequence stars we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, that the position of the observed field is close to optimal for recovering the initial mass function of the cluster stars (below the current turn-off mass). More generally we look at how the mass function changes with radius in a dynamically evolved stellar cluster and suggest where the best radial position to observe the initial mass function is for clusters of any age.

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