Category Archives: NGC6397

Globular cluster NGC6397

The WD Cooling Sequence of NGC6397

NGC 6397 :: new preprint

The White Dwarf Cooling Sequence of NGC6397 

(revised version)

Authors: Brad M. S. Hansen, Jay Anderson, James Brewer, Aaron Dotter, Greg. G. Fahlman, Jarrod Hurley, Jason Kalirai, Ivan King, David Reitzel, Harvey B. Richer, R.Michael Rich, Michael M. Shara, Peter B. Stetson

Comments: 56 pages, 30 figures

We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC6397, focussing attention on the cluster’s white dwarf cooling sequence. This sequence is shown to extend over 5 magnitudes in depth, with an apparent cutoff at magnitude F814W=27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in colour and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity and cooling model uncertainties. After marginalising over these variables, we obtain values for the cluster distance modulus and age of \mu_0 = 12.02 \pm 0.06 and T_c = 11.47 \pm 0.47Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relations are in good agreement with other independent determinations, and the cluster age is consistent with, but more precise than, prior determinations made using the main sequence turnoff method. In particular, within the context of the currently accepted \Lambda CDM cosmological model, this age places the formation of NGC6397 at a redshift z=3, at a time when the cosmological star formation rate was approaching its peak.


Probing the Faintest Stars in a Globular Star Cluster

New preprint :: NGC 6397

Probing the Faintest Stars in a Globular Star Cluster

Authors: Harvey B. Richer, Jay Anderson, James Brewer, Saul Davis, Gregory G. Fahlman, Brad M.S. Hansen, Jarrod Hurley, Jasonjot S. Kalirai, Ivan R. King, David Reitzel, R. Michael Rich, Michael M. Shara, Peter B. Stetson
Comments: 12 pages, 4 figures. Full Resolution Figures in Science, 2006, 313, 936
Journal-ref: Science (2006), 313, 936

NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed the deepest ever color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen-burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn towards bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H2 causes their atmospheres to become largely opaque to infrared radiation due to collision-induced absorption.


The Space Motion of NGC 6397

New preprint :: NGC 6397

The Space Motion of the Globular Cluster NGC 6397

Authors:  Jasonjot S. Kalirai, Jay Anderson, Harvey B. Richer, Ivan R. King, James P. Brewer, Giovanni Carraro, Saul D. Davis, Gregory G. Fahlman, Brad M. S. Hansen, Jarrod R. Hurley, Sebastien Lepine, David B. Reitzel, R. Michael Rich, Michael M. Shara, Peter B. Stetson
Comments: 5 pages including 3 figures, accepted for publication in the Astrophysical Journal Letters. Very minor changes in V2. typos fixed

 As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397’s UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.


Tidal disruption and the tale of three clusters

New preprint :: NGC6397, NGC 6712, NGC 6218

Tidal disruption and the tale of three clusters
Authors:  Guido De Marchi (ESA), Francesco Paresce (INAF), Luigi Pulone (INAF)
Comments: Two pages, one figure, to appear in the proceedings of “Globular Clusters – Guides to Galaxies”, eds. T. Richtler and S. Larsen

 How well can we tell whether a globular cluster will survive the Galaxy’s tidal forces? This is conceptually easy to do if we know the cluster’s total mass, mass structure and space motion parameters. This information is used in models that predict the probability of disruption due to tidal stripping, disc and bulge shocking. But just how accurate is the information that goes into these models and, therefore, how reliable are their predictions? To understand the virtues and weaknesses of these models, we have studied in detail three globular clusters (NGC 6397, NGC 6712, NGC 6218) whose predicted interaction with the galaxy is very different. We have used deep HST and VLT data to measure the luminosity function of stars throughout the clusters in order to derive a solid global mass function, which is the best tell-tale of the strength and extent of tidal stripping operated by the Galaxy. We indeed find that the global mass functions of the three clusters are different, but not in the way predicted by the models. [abridged]


HST astrometry and VLT radial velocities in NGC 6397

NGC 6397 :: New published paper

A. P. Milone et al.

Absolute motions of globular clusters

II. HST astrometry and VLT radial velocities in NGC 6397

In this paper we present a new, accurate determination of the three components of the absolute space velocity of the Galactic globular cluster NGC 6397 ( $l\simeq338^\circ$, $b\simeq-12^\circ$). We used three HST/WFPC2 fields with multi-epoch observations to obtain astrometric measurements of objects in three different fields in this cluster. The identification of 33 background galaxies with sharp nuclei allowed us to determine an absolute reference point and measure the absolute proper motion of the cluster. The third component was obtained from radial velocities measured from spectra from the multi-fiber spectrograph FLAMES at UT2-VLT. We find ( $\mu_\alpha \cos{\delta}$, $\mu_\delta$) $_{\rm J2000.0}$ = ( $+3.39 \pm 0.15$, $-17.55 \pm 0.15$) mas yr-1 and $V_{\rm rad} = +18.36 \pm 0.09$ ($\pm$0.10) km s-1. Assuming a Galactic potential, we calculate the cluster orbit for various assumed distances and briefly discuss the implications.

A&A 456 (2006) 517-522 (Section ‘Galactic structure, stellar clusters, and populations’)

Astronomy & Astrophysics website

Hubble Sees Faintest Stars in a Globular Cluster

NGC 6397 :: HST Press Release

Hubble Sees Faintest Stars in a Globular Cluster

NASA’s Hubble Space Telescope has uncovered what astronomers are reporting as the dimmest stars ever seen in any globular star cluster. Globular clusters are spherical concentrations of hundreds of thousands of stars.

These clusters formed early in the 13.7-billion-year-old universe. The cluster NGC 6397 is one of the closest globular star clusters to Earth. Seeing the whole range of stars in this area will yield insights into the age, origin, and evolution of the cluster.

Credit: NASA, ESA, and H. Richer (University of British Columbia)