Monthly Archives: August 2007

Papers on Omega Cen, NGC6440, NGC6388, NGC6441, M4…

Hello, here is a brief list (not sure if complete…) with the latest preprints regarding Galactic Globular Clusters, which appeared this month on astro-ph:

FSR 1767: New Milky Way Globular Cluster, No. 158

A new globular cluster in the Milky Way has just been announced: FSR 1767.

This one is particular in being the closest known globular to us at only about 4,900 light-years, thus beating M4 and NGC 6397 at about 6,500 to 7,500 ly. It is a low-mass globular with perhaps 1/10 of the stars in M4, and estimated as of absolute magnitude -4.7 Mv, diameter 6.5 ly.

According to my counting, this is currently Milky Way Globular Cluster No. 158 to be recognized. [- Hartmut]

Charles Bonatto, Eduardo Bica, Sergio Ortolani, Beatriz Barbuy, 2007. FSR1767 – a new globular cluster in the Galaxy. To be published in MNRAS.

The globular cluster (GC) nature of the recently catalogued candidate FSR 1767 is established in the present work. It results as the closest GC so far detected in the Galaxy. The nature of this object is investigated by means of 2MASS colour-magnitude diagrams (CMDs), the stellar radial density profile (RDP) and proper-motions (PM). The properties are consistent with an intermediate metallicity ($\feh\approx-1.2$) GC with a well-defined turnoff (TO), red-giant branch (RGB) and blue horizontal-branch (HB). The distance of FSR 1767 from the Sun is $\ds\approx1.5$ kpc, and it is located at the Galactocentric distance $\rgc\approx5.7$ kpc. With the space velocity components $(V,W)=(184\pm14,-43\pm14)\rm km s^{-1}$, FSR 1767 appears to be a Palomar-like GC with $\mv\approx-4.7$, that currently lies $\approx57$ pc below the Galactic plane. The RDP is well represented by a King profile with the core and tidal radii $\rc=0.24\pm0.08$ pc and $\rt=3.1\pm1.0$ pc, respectively, with a small half-light radius $\rh=0.60\pm0.15$ pc. The optical absorption is moderate for an infrared GC, $A_V=6.2\pm0.3$, which together with its central direction and enhanced contamination explains why it has so far been overlooked.


SEDS webpage:

ACS survey: M54 and Sagittarius dwarf galaxy

NGC6715 (M54) :: new preprint

The ACS Survey of Galactic Globular Clusters:
M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy

Authors: Michael H. Siegel, Aaron Dotter, Steven R. Majewski, Ata Sarajedini, Brian Chaboyer, David L. Nidever, Jay Anderson, Antonio Marín-Franch, Alfred Rosenberg, Luigi R. Bedin, Antonio Aparicio, Ivan King, Giampaolo Piotto, I. Neill Reid

Abstract: We present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy as part of the ACS Survey of Galactic Globular Clusters. Our deep (F606W~26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. The distance and reddening to M54 are revised usingboth isochrone and main-sequence fitting to (m-M)o=17.27 and E(B-V)=0.15. Preliminary assessment finds the M54+Sgr field to be dominated by the old metal-poor populations of Sgr and the globular cluster. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, 2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (0.1-0.8 Gyr old), more metal-rich ([Fe/H]\sim0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.

The intermediate-age cluster NGC 1783 in the LMC

Extragalactic GCs :: new preprint

The intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud:

Authors: A. Mucciarelli, L. Origlia , F. R. Ferraro

Abstract: We present Hubble Space Telescope ACS deep photometry of the intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud. By using this photometric dataset, we have determined the degree of ellipticity of the cluster (0.14 +/- 0.03) and the radial density profile. This profile is well reproduced by a standard King model with an extended core (rc=24.5”) and a low concentration (c=1.16), indicating that the cluster has not experienced the collapse of the core. We also derived the cluster age, by using the Pisa Evolutionary Library (PEL) isochrones, with three different amount of overshooting (namely, Lambda = 0.0, 0.10 and 0.25). From the comparison of the observed Color-Magnitude Diagram (CMD) and Main Sequence (MS) Luminosity Function (LF) with the theoretical isochrones and LFs, we find that only models with the inclusion of some overshooting (Lambda(os)=0.10-0.25) are able to reproduce the observables. By using the magnitude difference Delta V(SGB- He-Cl)=0.90 between the mean level of the He-clump and the flat region of the SGB, we derive an age equal to 1.4 +/- 0.2 Gyr.