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TOPIC: 47 Tucanae
May 4 04:40 2007
RE: 47 Tucanae
Zooming on the globular cluster 47 Tucanae in the constellation of Tucana
Dec 11 09:13 2006
Searching for Planetary Transits in Globular Clusters - 47 Tucanae and omega Centauri
David T.F Weldrake, Penny D Sackett, Terry J Bridges
We have performed a large ground-based search for transiting Hot Jupiter planets in the outer regions of the globular clusters 47 Tucanae and omega Centauri. The aim was to help understand the role that environmental effects play on Hot Jupiter formation and survivability in globular clusters. Using the ANU 1m telescope and a 52' X 52' field, a total of 54,000 solar-type stars were searched for transits in both clusters with fully tested transit-finding algorithms. Detailed Monte Carlo simulations were performed to model the datasets and calculate the expected planet yields. Seven planets were expected in 47 Tuc, and five in omega Cen. Despite a detailed search, no planet-like candidates were identified in either cluster. Combined with previous theoretical studies of planet survivability, and the HST null result in the core of 47 Tuc, the lack of detections in the uncrowded outer regions of both clusters indicates that stellar metallicity is the dominant factor inhibiting Hot Jupiter formation in the cluster environment.
Oct 24 17:29 2006
Globular Cluster 47 Tucanae
NASA's Hubble Space Telescope has provided astronomers with the best observational evidence to date that globular clusters sort out stars according to their mass, governed by a gravitational billiard ball game between stars. Heavier stars slow down and sink to the cluster's core, while lighter stars pick up speed and move across the cluster to its periphery. This process, called "mass segregation," has long been suspected for globular star clusters, but has never before been directly seen in action.
A typical globular cluster contains several hundred thousand stars. Although the density of stars is very small in the outskirts of such stellar systems, the stellar density near the centre can be more than 10,000 times higher than in the local vicinity of our Sun. If we lived in such a region of space, the night sky would be ablaze with 10,000 stars that would be closer to us than the nearest star to the Sun, Alpha Centauri, which is 4.3 light-years away (or approximately 215,000 times the distance between Earth and the Sun).
(130KB, 800 X 640)
Position (2000): R.A. 00h 24m 05s.67 Dec. -72° 04' 52".6
Oct 6 19:31 2006
Blue straggler stars
Astronomers have found evidence of one way a strange type of star called a "blue straggler" might form: by siphoning stellar material off a neighbour.
Blue stragglers are anomalous young stars found in globular clusters of much older stars. They appear to be bluer and hotter than their neighbours.
These stellar oddities have mystified astronomers, who don't know exactly how they form. Any stars that coalesced in a globular cluster should be the same age and similar in appearance, according to conventional thinking.
The two most likely scenarios astronomers have devised for their formation are stellar collisions and feeding off of older stars in a binary system. The new study, published in the Astrophysical Journal, found evidence of the second mechanism.
to read more.
Oct 2 16:54 2006
Stellar Vampires Unmasked
VLT Presents Evidence for Mass Transfer as Origin of some Blue Straggler Stars
Astronomers have found possible proofs of stellar vampirism in the globular cluster 47 Tucanae. Using ESO's Very Large Telescope, they found that some hot, bright, and apparently young stars in the cluster present less carbon and oxygen than the majority of their sisters. This indicates that these few stars likely formed by taking their material from another star.
This is the first detection of a chemical signature clearly pointing to a specific scenario to form so-called 'Blue straggler stars' in a globular cluster
" - Francesco Ferraro, from the Astronomy Department of Bologna University (Italy) and lead-author of the paper presenting the results.
Blue stragglers are unexpectedly young-looking stars found in stellar aggregates, such as globular clusters, which are known to be made up of old stars. These enigmatic objects are thought to be created in either direct stellar collisions or through the evolution and coalescence of a binary star system in which one star 'sucks' material off the other, rejuvenating itself. As such, they provide interesting constraints on both binary stellar evolution and star cluster dynamics. To date, the unambiguous signatures of either stellar traffic accidents or stellar vampirism have not been observed, and the formation mechanisms of Blue stragglers are still a mystery.
The astronomers used ESO's Very Large Telescope to measure the abundance of chemical elements at the surface of 43 Blue straggler stars in the globular cluster 47 Tucanae.
They discovered that six of these Blue straggler stars contain less carbon and oxygen than the majority of these peculiar objects. Such an anomaly indicates that the material at the surface of the blue stragglers comes from the deep interiors of a parent star. Such deep material can reach the surface of the blue straggler only during the mass transfer process occurring between two stars in a binary system. Numerical simulations indeed show that the coalescence of stars should not result in anomalous abundances.
Jul 5 08:07 2006
RE: 47 Tucanae
Tracing the Dynamical History of the Globular Cluster 47 Tucanae
Eric Monkman, Alison Sills (McMaster University), Justin Howell (IPAC), Puragra Guhathakurta (Lick), Francesca de Angeli (IoA), Giacomo Beccari (Bologna, Teramo)
Researchers use two stellar populations in the globular cluster 47 Tucanae to trace its dynamical history: blue stragglers and low mass main sequence stars. They assumed that the blue stragglers were formed through stellar collisions in all regions of the cluster. They find that in the core of the cluster, models of collisional blue stragglers agree well with the observations as long as blue stragglers are still continuing to form and the mass function in the cluster is extremely biased towards massive stars (x=-8 where a Salpeter mass function has x=+1.35).
The researchers show that such an extreme mass function is supported by direct measurements of the luminosity function of main sequence stars in the centre of the cluster. In the middle region of their dataset (25'' to 130'' from the cluster centre), blue straggler formation seems to have stopped about half a billion years ago. In the outskirts of the cluster, their models are least successful at reproducing the blue straggler data. Taken at face value, they indicate that blue straggler formation has been insignificant over the past billion years, and that a Salpeter mass function applies. However, it is more likely that the dominant formation mechanism in this part of the cluster is not the collisional one, and that their models are not appropriate for this region of the cluster.
They conclude that blue stragglers can be used as tracers of dynamics in globular clusters, despite their incomplete understanding of how and where they were formed.
Sep 5 09:50 2005
Of the over 200 globular star clusters that orbit the centre of our Milky Way Galaxy,
, is the second brightest globular cluster, behind Omega Centauri.
47 Tuc is 20,000 light years away and can be seen near the
Small Magellanic Cloud
toward the constellation of Tucana. It is only visible from Earth's Southern Hemisphere.
(79kb, 766 x 600)
47 Tuc was the target for first light observations of the gigantic new 10-meter diameter Southern African Large Telescope (SALT) last week.
The dynamics of stars near the centre of 47 Tuc are not well understood, particularly why there are so few binary systems there.
-- Edited by Blobrana at 02:05, 2007-11-21
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