[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