Author Archives: Hartmut

FSR 584: Another new nearby globular cluster candidate

Bica et.al. have announced another nearby globular cluster candidate in the Milky Way:

http://arxiv.org/abs/0709.3315

Author: E. Bica, C. Bonatto, S. Ortolani, B. Barbuy
Title: FSR584 – a new globular cluster in the Galaxy?
Comments: Astronomy and Astrophysics, accepted. 6 pages and 6 figures

Abstract: We investigate the nature of the recently catalogued star cluster candidate FSR584, which is projected in the direction of the molecular cloud W3 and may be the nearest globular cluster to the Sun. 2MASS CMDs, the stellar radial density profile, and proper motions are employed to derive fundamental and structural parameters. The CMD morphology and the radial density profile show that FSR584 is an old star cluster. With proper motions, the properties of FSR584 are consistent with a metal-poor globular cluster with a well-defined turnoff and evidence of a blue horizontal-branch. FSR584 might be a Palomar-like halo globular cluster that is moving towards the Galactic plane. The distance from the Sun is approx 1.4kpc, and it is located at approx 1kpc outside the Solar circle. The radial density profile is characterized by a core radius of rc=0.3+/-0.1 pc. However, we cannot exclude the possibility of an old open cluster. Near-infrared photometry coupled to proper motions support the scenario where FSR584 is a new globular cluster in the Galaxy. The absorption is A_V=9.2+/-0.6$, which makes it a limiting object in the optical and explains why it has so far been overlooked

This one is apparently projected over starforming region W3, and at a distance of only 1.4 kpc (4,600 ly). It would probably even beat the recently discovered record holder, FSR 1767. However, as of this time, the authors cannot exclude that this may be an old open cluster.

A preliminary webpage for this probable globular has been created at

http://www.seds.org/~spider/spider/MWGC/fsr0584.html

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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]

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

Abstract:
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.

Preprint:
http://www.arxiv.org/abs/0708.0501

SEDS webpage:
http://www.seds.org/~spider/spider/MWGC/fsr1767.html

Image-Subtraction Photometry of Variable Stars in NGC 6388 and NGC 6441

New paper (“The Astronomical Journal”, Volume 132):

    Corwin, T. Michael; Sumerel, Andrew N.; Pritzl, Barton J.; Smith, Horace A.; Catelan, M.; Sweigart, Allen V.; Stetson, Peter B.

Image-Subtraction Photometry of Variable Stars in the Globular Clusters NGC 6388 and NGC 6441

We have applied Alard’s image-subtraction method (ISIS ver. 2.1) to the observations of the globular clusters NGC 6388 and NGC 6441 previously analyzed using standard photometric techniques (DAOPHOT and ALLFRAME). In this reanalysis of observations obtained at the Cerro Tololo Inter-American Observatory, besides recovering the variables previously detected on the basis of our ground-based images, we have also been able to recover most of the RR Lyrae variables previously detected only in the analysis of Hubble Space Telescope WFPC2 observations of the inner region of NGC 6441. In addition, we report five possible new variables not found in the analysis of the Hubble Space Telescope observations of NGC 6441. This dramatically illustrates the capabilities of image-subtraction techniques applied to ground-based data to recover variables in extremely crowded fields. We have also detected 12 new variables and 6 possible variables in NGC 6388 not found in our previous ground-based studies. The revised mean period for RRab stars in NGC 6388 is 0.676 days, while the mean period of RRab stars in NGC 6441 is unchanged at 0.759 days. These values are among the largest known for any Galactic globular cluster. Additional probable type II Cepheids were identified in NGC 6388, confirming its status as a metal-rich globular cluster rich in Cepheids.


ADS link to the paper

(mcastel)

On the primordial scenario for abundance variations within GCs…

New paper (ApJ Vol. 645):


Salaris, Maurizio; Weiss, Achim; Ferguson, Jason W.; Fusilier, David J.

   
On the Primordial Scenario for Abundance Variations within Globular Clusters: The Isochrone Test

Self-enrichment processes occurring in the early stages of a globular cluster lifetime are generally invoked to explain the observed CNONaMgAl abundance anticorrelations within individual Galactic globular clusters. We have tested, with fully consistent stellar evolution calculations, whether theoretical isochrones for stars born with the observed abundance anticorrelations satisfy the observational evidence that objects with different degrees of these anomalies lie on essentially identical sequences in the color-magnitude diagram (CMD). To this purpose, we have computed for the first time low-mass stellar models and isochrones with an initial metal mixture that includes the extreme values of the observed abundance anticorrelations and varying initial He mass fractions. Comparisons with “normal” α-enhanced isochrones and suitable Monte Carlo simulations that include photometric errors show that a significant broadening of the CMD sequences occurs only if the helium enhancement is extremely large (in this study, when Y=0.35) in the stars showing anomalous abundances. Stellar luminosity functions up to the red giant branch tip are also very weakly affected, apart from-depending on the He content of the polluting material-the red giant branch bump region. We also study the distribution of stars along the zero-age horizontal branch and derive general constraints on the relative location of objects with and without abundance anomalies along the observed horizontal branches of globular clusters.

Publication: The Astrophysical Journal, Volume 645, Issue 2, pp. 1131-1137.

ADS Link

astro-ph Link