New address for this blog...

Galactic Globular Clusters Blog has moved!

The new location can be reached at this link

VLT Image of a Unique Swarm of Stars, 47 Tuc

[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....


http://www.eso.org/outreach/press-rel/pr-2006/pr-20-06.html

Abundance anomalies in hot HB stars of NGC 2808

[Paper]

Pace, G.; Recio-Blanco, A.; Piotto, G.; Momany, Y.

Abundance anomalies in hot horizontal branch stars of the Galactic globular cluster NGC 2808

Astronomy and Astrophysics, Volume 452, Issue 2, June III 2006, pp.493-501

Aims.We present metallicity measurements of 25 stars in the blue horizontal branch of the Galactic globular cluster NGC 2808.
Methods: . Our measurements are based on moderate-resolution spectra taken with the multi-object fiber facility FLAMES-UVES, mounted on Kueyen at the Very Large Telescope.
Results: . We confirm that stars hotter than a threshold temperature have super-solar abundance, while the cooler ones respect the nominal metallicity of the cluster, i.e. [Fe/H]≃-1.1. The threshold temperature is estimated to be about 12 000 K, corresponding to the so called u-jump, and coincides with the sudden departure of the cluster horizontal branch from the models. The metallicity increases with temperature for star hotter than the jump, confirming the hypothesis that the process responsible for this abrupt metallic enhancement is the levitation due to the strong radiation field in absence of a significative convective envelope. A metallicity dependence of the abundance enhancement is also suggested, with more metal poor clusters having a higher increase in metal content.
Conclusions: .The slope in the temperature vs. abundance diagram is higher than the errors involved, and the metal content of the cluster plays possibly a role in determining the amplitude of the jump (more metal poor clusters show more enhancement after the jump), although other parameters, such as clusters' characteristics and even the atomic species involved, may also someway contribute.

ADS Link

Preprints: W. Miller, F. Schweizer

Preprints:

Globular Clusters in Dwarf Galaxies

Author: Bryan W. Miller (Gemini Observatory)

Recent work on globular cluster systems in dwarf galaxies outside the Local Group is reviewed. Recent large imaging surveys with the Hubble Space Telescope and follow-up spectroscopy with 8-m class telescopes now allow us to compare the properties of massive star clusters in a wide range of galaxy types and environments. This body of work provides important constraints for theories of galaxy and star cluster formation and evolution.

http://arxiv.org/abs/astro-ph/0606062

---

Globular Cluster Formation in Mergers
Author: Francois Schweizer (Carnegie Observatories, Pasadena)

Mergers of gas-rich galaxies lead to gravitationally driven increases in gas pressure that can trigger intense bursts of star and cluster formation. Although star formation itself is clustered, most newborn stellar aggregates are unbound associations and disperse. Gravitationally bound star clusters that survive for at least 10-20 internal crossing times (~20-40 Myr) are relatively rare and seem to contain <10% of all stars formed in the starbursts. The most massive young globular clusters formed in present-day mergers exceed omega Cen by an order of magnitude in mass, yet appear to have normal stellar initial mass functions. (...)


http://arxiv.org/abs/astro-ph/0606036

Dedication...

Just a line, to let you know that the "Globular Clustes Database" from now on, is dedicated to my father, Vittorio Castellani, who passed away on May 20, 2006.

He told me several times to keep on on this project, so... daddy, these pages are for you: from where you are now, please from time to time, take a look and see if I'm doing a "good job" ... ;)

Marco Castellani

New page for cluster GLIMPSE-C01

Added in the database the "new" cluster GLIMPSE-C01

http://www.mporzio.astro.it/~marco/gc/cluster_4.php?ggc=GLIMPSE-C01

Greetings,

Marco

New feature: improvement of "Top Fifty" page

The "Top Fifty" webpage now has direct links to all listed cluster

http://www.mporzio.astro.it/~marco/gc/vclusters.php

The same is true also for the little "preview" box available in the main page.

Greetings,

Marco

The Formation Histories of Metal-Rich and Metal-Poor Globular Clusters

Preprint (astro-ph/0605373)

Authors: Stephen E. Zepf
The Formation Histories of Metal-Rich and Metal-Poor Globular Clusters

To appear in the proceedings of the "Globular Clusters: Guides to Galaxies" conference

Abstract:
This review presents the results of ongoing studies of the formation histories of metal-poor and metal-rich globular clusters and their host galaxies. I first discuss the strong observational evidence that the globular cluster systems of most elliptical galaxies have bimodal metallicity distributions. I then focus on new results for metal-poor and metal-rich globular cluster systems. Metal-poor globular clusters are often associated with early structure formation, and I review new constraints on their formation epoch based on the ``bias'' of the number of metal-poor clusters with host galaxy mass. For metal-rich globular clusters, I discuss new results from ongoing optical to near-infrared photometric studies which both confirm an intermediate-age population in NGC 4365 and generally reveal a variety of formation histories for now quiescent ellipticals.


http://babbage.sissa.it/abs/astro-ph/0605373

[preprint] Globular clusters and galaxy formation

Duncan A. Forbes
Globular clusters and galaxy formation

We first discuss recent progress in using the Milky Way globular cluster (GC) system as a `test-bed' for properties derived from integrated spectra and stellar population models. Standard techniques may give rise to spuriously high alpha-element ratios at low metallicities. We then discuss evidence for early epoch (z > 2) formation for most GCs in galaxies today. Recent accretions of GCs (and their host galaxy) make a small contribution but recent mergers form few if any new GCs in today's elliptical galaxies. The early formation of metal-poor GCs and the bimodality seen in GC specific frequency requires a `truncation' which may be due to reionization.

(Globular Clusters - Guides to Galaxies conference)

http://arxiv.org/abs/astro-ph/0605209

[Preprint] Globular Cluster System evolution in early type galaxies

Preprint

R. Capuzzo-Dolcetta
"Globular Cluster System evolution in early type galaxies"

Globular clusters (GCs) constitute a system which is evolving because of various interactions with the galactic environment. Evolution may be the explanation of many observed features of Globular Cluster Systems (GCSs); the different radial distribution of the GCS and the stellar component of early type galaxies is explained by dynamical friction and tidal effects, this latter acting both on the large scale (that of the bulge-halo stars) and on the small scale (that of the nucleus, often containing a central massive black hole). Merging of quickly orbitally decayed massive GCs leads to formation of a Super Star Cluster (SSC) which enriches the galactic nucleus and is a reservoire of mass-energy for a centrally located black hole

Talk given at the Globular Clusters Guide to Galaxies conference held in Concepcion (Chile) march 6-10 2006


http://arxiv.org/abs/astro-ph/0605162

[paper] CCD Photometry of the Globular Cluster ESO 452-SC11

Cornish, Alexis S. M.; Phelps, Randy L.; Briley, Michael M.; Friel, Eileen D.

"CCD Photometry of the Globular Cluster ESO 452-SC11"

The globular cluster ESO 452-SC11 has been observed using Johnson V and Cousins I filters, and point-spread function photometry has been performed. The resulting color-magnitude diagrams were compared to theoretical isochrones to derive the cluster's age, overall chemical composition, and distance modulus. These isochrone models include those published by Girardi et al. and Bergbusch & VandenBerg, with BVRI color-Teff relations as described by VandenBerg & Clem, and Demarque et al. (known as the Yale isochrones). From the Yale isochrones, it is estimated that the cluster has an age of 9-11 Gyr, a metallicity -1.4 dex<=[Fe/H]<=-1.0 dex, and a distance modulus (m-M)V=16.10-16.31 mag, resulting in a heliocentric distance of 7.3-7.5 kpc...

The Astronomical Journal, Volume 131, Issue 5, pp. 2543-2550

ADS Link

[paper] Globular cluster system and Milky Way properties revisited

E. Bica - C. Bonatto - B. Barbuy - S. Ortolani

Globular cluster system and Milky Way properties revisited

"Aims. Updated data of the 153 Galactic globular clusters are used to readdress fundamental parameters of the Milky Way, such as the distance of the Sun to the Galactic centre, the bulge and halo structural parameters, and cluster destruction rates.
Methods. We build a reduced sample that has been decontaminated of all the clusters younger than 10 Gyr and of those with retrograde orbits and/or evidence of relation to dwarf galaxies...."

Astronomy and Astrophysics, Volume 450, Issue 1, April IV 2006, pp.105-115


ADS link to the paper

[paper] Probable member stars of the gravitational theory-testing globular clusters AM 1, Pal 3 and Pal 14

Hilker, M.
Probable member stars of the gravitational theory-testing globular clusters AM 1, Pal 3 and Pal 14


Astronomy and Astrophysics, Volume 448, Issue 1, March II 2006, pp.171-180

"Some of the Galactic outer halo globular clusters are excellent tools to probe gravitational theories in the regime of weak accelerations. The measurement of the line-of-sight velocity dispersion among stars in these clusters will differentiate between the validity of Newtonian dynamics (low velocity dispersion) and the possiblity of modified Newtonian dynamics (MOND) or dark matter dominated globular clusters (high velocity dispersion). In this paper, the properties of probable member stars of the three best-case gravitational theory-testing clusters AM 1, Pal 3 and Pal 14 are presented...."


ADS link

[Press release] A River of Stars Streaming Across the Northern Sky

Astronomers Discover a River of Stars Streaming Across the Northern Sky

PASADENA, Calif.--Astronomers have discovered a narrow stream of stars extending at least 45 degrees across the northern sky. The stream is about 76,000 light-years distant from Earth and forms a giant arc over the disk of the Milky Way galaxy...The stream emanates from a cluster of about 50,000 stars known as NGC 5466.

http://pr.caltech.edu/media/Press_Releases/PR12811.html

[preprint] Abundance anomalies in NGC 2808

NGC 2808 :: Preprint

Abundance anomalies in NGC 2808

Authors: G. Pace, A. Recio-Blanco, G. Piotto, Y. Momany
Comments: 11 pages, 7 figures, accepted by A&A

"We present metallicity measurements of 25 stars in the blue horizontal branch of the Galactic globular cluster NGC 2808. Our measurements are based on moderate-resolution spectra taken with the multi-object fiber facility FLAMES-UVES, mounted on Kueyen at the Very Large Telescope. We confirm that stars hotter than a threshold temperature have super-solar abundance, while the cooler ones respect the nominal metallicity of the cluster, i.e. [Fe/H] ~ -1.1...."

URL: http://babbage.sissa.it/abs/astro-ph/0602629

[preprint] Detection of a 45 Degree Tidal Stream Associated with the GC NGC 5466

NGC 5466 :: preprint

C. J. Grillmair, R. Johnson

"Detection of a 45 Degree Tidal Stream Associated with the Globular Cluster NGC 5466"

We report on the detection in Sloan Digital Sky Survey data of a 45 degree tidal stream of stars, extending from Bootes to Ursa Major, which we associate with the halo globular cluster NGC 5466. Using an optimal contrast, matched filter technique, we find a long, almost linear stellar stream with an average width of 1.4 degrees. The stream is an order of magnitude more tenuous than the stream associated with Palomar 5. The stream's orientation on the sky is consistent to a greater or lesser extent with existing proper motion measurements for the cluster.

URL : http://babbage.sissa.it/abs/astro-ph/0602602

[Press release] How to Steal a Million Stars?

[ESO press release]

How to Steal a Million Stars?
VLT Study Reveals Troubled Past of Globular Cluster Messier 12

Based on observations with ESO's Very Large Telescope, a team of Italian astronomers reports that the stellar cluster Messier 12 must have lost to our Milky Way galaxy close to one million low-mass stars.

ESO PR Photo 04a/06 The Central Part of Messier 12

"In the solar neighbourhood and in most stellar clusters, the least massive stars are the most common, and by far", said Guido De Marchi (ESA), lead author of the study. "Our observations with ESO's VLT show this is not the case for Messier 12."

The team, which also includes Luigi Pulone and Francesco Paresce (INAF, Italy), measured the brightness and colours of more than 16,000 stars within the globular cluster Messier 12 with the FORS1 multi-mode instrument attached to one of the Unit Telescopes of ESO's VLT at Cerro Paranal (Chile). The astronomers could study stars that are 40 million times fainter than what the unaided eye can see (magnitude 25).

http://www.eso.org/outreach/press-rel/pr-2006/pr-04-06.html

[preprint] New Metallicities of RR Lyrae Stars in omega Centauri...

[NGC5139 (ω Centauri)] New preprint:

A. Sollima, J. Borissova, M. Catelan, H. A. Smith, D. Minniti, C. Cacciari, F. R. Ferraro

"New Metallicities of RR Lyrae Stars in omega Centauri: Evidence for a Non He-Enhanced Metal-Intermediate Population"

Abstract:

"We present new spectroscopic metal abundances for 74 RR Lyrae stars in omega Cen obtained with FLAMES. The well-known metallicity spread is visible among the RR Lyrae variables. The metal-intermediate (MInt) RR Lyrae stars ([Fe/H] ~ -1.2) are fainter than the bulk of the dominant metal-poor population ([Fe/H] ~ -1.7), in good agreement with the corresponding zero-age horizontal branch models with cosmological helium abundance Y = 0.246. This result conflicts with the hypothesis that the progenitors of the MInt RR Lyrae stars correspond to the anomalous blue main-sequence stars, which share a similar metallicity but whose properties are currently explained by assuming for them a large helium enhancement. Therefore, in this scenario, the coexistence within the cluster of two different populations with similar metallicities ([Fe/H] ~ -1.2) and different helium abundances has to be considered."

http://arxiv.org/abs/astro-ph/0602055

[paper] The Origin of the Double Main Sequence in ω Centauri: Helium Enrichment due to Gas Fueling from Its Ancient Host Galaxy?

[NGC5139 (ω Centauri)] New paper:

Bekki, Kenji; Norris, John E.

" The Origin of the Double Main Sequence in ω Centauri: Helium Enrichment due to Gas Fueling from Its Ancient Host Galaxy?"

Abstract:
Recent observational studies of ω Centauri by the Hubble Space Telescope have discovered a double main sequence in the color-magnitude diagrams (CMDs) of its stellar populations. These observations suggest that the stellar population with the blue main sequence (bMS) has a helium abundance much larger, by ΔY~0.12, than that of the red main sequence (rMS). By using somewhat idealized models in which stars of the bMS are formed from gas ejected from stars of the rMS, we quantitatively investigate whether the helium overabundance of the bMS can result from self-enrichment from massive AGB stars, from mass loss of very massive young stars, or from Type II supernovae within ω Cen. We show that as long as the helium enrichment is due to ejecta from the rMS formed earlier than the bMS, none of these three enrichment scenarios can explain the observed properties of the bMS self-consistently for reasonable IMFs. The common, serious problem in all cases is that the observed number fraction of the bMS cannot be explained without assuming unusually top-heavy IMFs. This failure of the self-enrichment scenarios implies that most of the helium-enriched gas necessary for the formation of the bMS originated from other external sources. We thus suggest a new scenario, in which most of the second generation of stars (i.e., the bMS) in ω Cen could have formed from gas ejected from field stellar populations that surrounded ω Cen when it was the nucleus of an ancient dwarf galaxy.

The Astrophysical Journal, Volume 637, Issue 2, pp. L109-L112.

ADS Link to the paper

NGC2808 :: New paper

NGC2808 :: New paper

Castellani, V.; Iannicola, G.; Bono, G.; Zoccali, M.; Cassisi, S.; Buonanno, R.

On the horizontal branch of the galactic globular cluster NGC 2808

(Astronomy and Astrophysics, Volume 446, Issue 2, February I 2006, pp.569-577 )

Abstract:

We present new UV (F218W) data for stars in the central region of the Galactic globular cluster NGC 2808, collected with the WFPC2 camera on board the Hubble Space Telescope. These data together with F439W and F555W-band data and previous ground-based observations provide a multifrequency coverage of the cluster stellar population extending up to a distance of 1.7 times the cluster core radius. We discuss this complete sample of stars, which includes 764 Red Giant Branch (RGB) stars brighter than the Horizontal Branch (HB) luminosity level, 1239 HB stars, 119 Asymptotic Giant Branch (AGB), and 22 AGB-manqué stellar structures. As already known, we find that blue HB stars separate into three distinct groups. However, our multiband photometry indicates that several stars in the two hotter HB groups show a flat spectrum, thus suggesting the binarity of these objects. Artificial star experiments suggest that at most 50% of them might be photometric blends. Moreover, at variance with previous claims one finds that canonical Zero Age Horizontal Branch (ZAHB) models do reach effective temperatures typical of observed hot HB stars. We also show that the ratio between HB and RGB stars brighter than the HB luminosity level steadly increases when moving from the cluster center to the periphery, passing from R=1.37±0.14 in the cluster core to R=1.95±0.26 in the outer cluster regions. We discuss the possible origin of such a radial gradient in the context of the Blue Tails phenomenon, advancing some suggestions concerning the clumpy stellar distribution along the HB.

Upper limits on the central black hole masses of 47Tuc and NGC6397 (preprint)

[NGC104, NGC6397] New preprint:

S. De Rijcke, P. Buyle, H. Dejonghe

"Upper limits on the central black hole masses of 47Tuc and NGC6397"

4 pages, 1 figure, accepted for publication by MNRAS

"We present upper-limits on the masses of the putative central intermediate-mass black holes in two nearby Galactic globular clusters: 47Tuc (NGC104), the second brightest Galactic globular cluster, and NGC6397, a core-collapse globular cluster and, with a distance of 2.7 kpc, quite possibly the nearest globular cluster, using a technique suggested by T. Maccarone. These mass estimates have been derived from 3sigma upper limits on the radio continuum flux at 1.4 GHz, assuming that the putative central black hole accretes the surrounding matter at a rate between 0.1% and 1% of the Bondi accretion rate. For 47Tuc, we find a 3sigma upper limit of 2060 - 670 solar masses, depending on the actual accretion rate of the black hole and the distance to 47Tuc. For NGC6397, which is closer to us, we derive a 3sigma upper limit of 1290 - 390 solar masses..."

http://arxiv.org/abs/astro-ph/0601450

Deep Photometry of the Globular Cluster M5...

[M5 (NGC5904)] One paper added to the database:

Layden, Andrew C.; Sarajedini, Ata; von Hippel, Ted; Cool, Adrienne M.

"Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main-Sequence Stars "

The Astrophysical Journal, Volume 632, Issue 1, pp. 266-276. (2005)

"We present deep VI photometry of stars in the globular cluster M5 (NGC 5904) based on images taken with the Hubble Space Telescope. The resulting color-magnitude diagram reaches below V~27 mag, revealing the upper 2-3 mag of the white dwarf cooling sequence and main-sequence stars 8 mag and more below the turnoff. We fit the main sequence to subdwarfs of known parallax to obtain a true distance modulus of (m-M)₀=14.45+/-0.11 mag. A second distance estimate based on fitting the cluster white dwarf sequence to field white dwarfs with known parallax yielded (m-M)₀=14.67+/-0.18 mag. We discuss the nature of the difference between the two distance estimates and suggest approaches for reducing the uncertainty in white dwarf fitting estimates for future studies. We couple our distance estimates with extensive photometry of the cluster's RR Lyrae variables to provide a calibration of the RR Lyrae absolute magnitude yielding M_V(RR)=0.42+/-0.10 mag at [Fe/H]=-1.11 dex. We provide another luminosity calibration in the form of reddening-free Wasenheit functions. Comparison of our calibrations with predictions based on recent models combining stellar evolution and pulsation theories shows encouraging agreement, and the existing differences may provide useful feedback to the models."

• ADS link

• GGC-DB link

NGC5139 (ω Centauri) - New paper

van de Ven, G.; van den Bosch, R. C. E.; Verolme, E. K.; de Zeeuw, P. T.

"The dynamical distance and intrinsic structure of the globular cluster ω Centauri"

"We determine the dynamical distance D, inclination i, mass-to-light ratio M/L and the intrinsic orbital structure of the globular cluster ω Cen, by fitting axisymmetric dynamical models to the ground-based proper motions of van Leeuwen et al. and line-of-sight velocities from four independent data-sets. We bring the kinematic measurements onto a common coordinate system, and select on cluster membership and on measurement error. This provides a homogeneous data-set of 2295 stars with proper motions accurate to 0.20 mas yr-1 and 2163 stars with line-of-sight velocities accurate to 2 km s-1, covering a radial range out to about half the tidal radius..."

• ADS Link

• GGC-DB link

Preprint: "An Inverse Look at the Center of M15"

[NGC7078 (M15)] Added on preprint to the database:

Dalia Chakrabarty
"An Inverse Look at the Center of M15"
Accepted for publication in AJ; 6 figures

"The observed radial and transverse velocities of individual stars in M15 are implemented as inputs to a fully non-parametric code (CHASSIS) in order to estimate the equilibrium stellar distribution function and the three-dimensional mass density profile. In particular, the paper explores the possibility of the existence of a central black hole in M15 via several runs that utilize the radial velocity data set which offers kinematic measurements closer to the centre of the cluster than the proper motion data..."

http://arxiv.org/abs/astro-ph/0601206

GGC-DB bibliography: new preprint

[Milky Way] Added one preprint to the database:

Francesco R. Ferraro
"Exotic populations in Galactic Globular Clusters"

Invited review in "Resolved Stellar Populations" 2005, Cancun, Mexico

"Recent high-resolution observations of the central region of Galactic globular clusters have shown the presence of a large variety of exotic stellar objects whose formation and evolution may be strongly affected by dynamical interactions. In this paper I review the main properties of two classes of exotic objects: the so-called Blue Stragglers stars and the recently identified optical companions to Millisecond pulsar.
Both these class of objects are invaluable tools to investigate the binary evolution in very dense environments and are powerful tracers of the dynamical history of the parent cluster."

http://babbage.sissa.it/abs/astro-ph/0601217

New pages on the "GGCs System"

I added a couple of pages related to the Galactic Globular Clusters System (as a whole).

There is new page with links to resources related to the Milky Way and GGCs System, at the URL:

http://www.mporzio.astro.it/~marco/gc/linksmw.php

and a page which hosts a selected bibliography (work is in progress!), at the URL:

http://www.mporzio.astro.it/~marco/gc/bibliomain.php

Both these new pages have links starting from the GGC-DB main page.


Greetings,

Marco C.

New feature: "Top fifty" webpage

I just added a very simple page with the ranking (according to the total number of visit) for the 50 "most popular" clusters. Direct access to the page is possible via the following link:

http://www.mporzio.astro.it/~marco/gc/vclusters.php

or, clicking to "Most visited pages" link in the GGC-DB main page.

I'd like to display the number of visit of a given period (say, six months...) instead of "total" number - i.e. in a way similar to the "page hit ranking" of Linux distros in Distrowatch.com, but it's still a bit tricky for my knowledge of PHP, for now ... :-)

Note that names of the clusters will be made "clickable" in the first improvement of this new page.

Greetings,

Marco