Monthly Archives: June 2006

Preprints on AL3, NGC6541, NGC6293

For all globular clusters fans…Two interesting papers on astro-ph today!

S. Ortolani, E. Bica, B. Barbuy

“AL 3 (BH 261): a new globular cluster in the Galaxy”

AL~3 (BH 261), previously classified as a faint open cluster candidate, is shown to be a new globular cluster in the Milky Way, by means of B, V and I Color-Magnitude Diagrams. The main feature of AL~3 is a prominent blue extended Horizontal Branch. Its Color-Magnitude Diagrams match those of the intermediate metallicity cluster M~5. The cluster is projected in a rich bulge field, also contaminated by the disk main sequence. (…) AL~3 is probably one of the least massive globular clusters of the Galaxy.

Jae-Woo Lee, Bruce W. Carney
“VI Photometry of Globular Clusters NGC6293 and NGC6541: The Formation of the Metal-Poor Inner Halo Globular Clusters”

We present VI photometry of the metal-poor inner halo globular clusters NGC6293 and NGC6541 using the planetary camera of the WFPC2 on board HST (…) The most notable result of our study is that the inner halo clusters NGC6293 and NGC6541 essentially have the same ages as M92, confirming the previous result from the HST NIC3 observations of NGC6287.


A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in 47 Tuc

New article on ApJ:

Heinke, Craig O.; Rybicki, George B.; Narayan, Ramesh; Grindlay, Jonathan E.
“A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae”

Current X-ray missions are providing high-quality X-ray spectra from neutron stars (NSs) in quiescent low-mass X-ray binaries (qLMXBs). This has motivated us to calculate new hydrogen atmosphere models, including opacity due to free-free absorption and Thomson scattering, thermal electron conduction, and self-irradiation by photons from the compact object. We have constructed a self-consistent grid of neutron star models covering a wide range of surface gravities, as well as effective temperatures, which we make available to the scientific community. We present multiepoch Chandra X-ray observations of the qLMXB X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on timescales from minutes to years. Its high-quality X-ray spectrum is adequately fitted by our hydrogen atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 Msolar is assumed, our spectral fits require that its radius be in the range Rns=14.5+1.8-1.6 km (90% confidence), which is larger than that expected from currently preferred models of NS interiors. If its radius is assumed to be 10 km, then a mass of Mns=2.20+0.03-0.16 Msolar is required. Using models with the appropriate surface gravity for each value of the mass and radius becomes important for interpretation of the highest quality data.

ADS link

Merging of Globular Clusters in Inner Galactic Regions. I.

    Miocchi, P.; Capuzzo Dolcetta, R.; Di Matteo, P.; Vicari, A.

Merging of Globular Clusters in Inner Galactic Regions. I. Do They Survive the Tidal Interaction?

The Astrophysical Journal, Volume 644, Issue 2, pp. 940-953.

The main topic of this paper is the investigation of the modes of interaction of globular clusters (GCs) moving in the inner part of a galaxy. This is tackled by means of high-resolution N-body simulations, whose first results are presented in this article. Our simulations dealt with primordial, very massive (of the order of 107 Msolar) GCs that were able to decay, because of dynamical friction, into the inner regions of triaxial galaxies on a timescale much shorter than their internal relaxation time….

ADS Link

VLT Image of a Unique Swarm of Stars, 47 Tuc

Free Image Hosting at

[link :: 47 Tuc]

The Southern constellation Tucana (the Toucan) is probably best known as the home of the Small Magellanic Cloud, one of the satellite galaxies of the Milky Way. But Tucana also hosts another famous object that shines thousands of lights, like a magnificent, oversized diamond in the sky: the globular cluster 47 Tucanae. More popularly known as 47 Tuc, it is surpassed in size and brightness by only one other globular cluster, Omega Centauri….