Searching for Planetary Transits in GCs…

NGC104 :: NGC5139 :: new preprint

Searching for Planetary Transits in Globular Clusters – 47 Tucanae and omega Centauri

Authors: David T.F Weldrake, Penny D Sackett, Terry J Bridges
Comments: 6 pages, 2 figures. To appear in the PASP proceedings of “Transiting Extrasolar Planets Workshop” MPIA Heidelberg Germany, 25th-28th September 2006. Eds: Cristina Afonso, David Weldrake & Thomas Henning

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.

URL: http://arxiv.org/abs/astro-ph/0612215

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Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tuc…

47 Tucanae (NGC 104) :: New published paper

The Astrophysical Journal Letters
Volume 647, Number 1
(August 10 2006)

F. R. Ferraro, E. Sabbi, R. Gratton, G. Piotto, B. Lanzoni, E. Carretta, R. T. Rood, Alison Sills, F. Fusi Pecci, S. Moehler, G. Beccari, S. Lucatello, and N. Compagni
“Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process”

Page L53 [ http://www.journals.uchicago.edu/cgi-bin/resolve?ApJL20841 ]

Abstract:
We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 blue straggler stars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs shows a significant depletion of carbon and oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.

Table of Contents of ApL Volume 647, Number 1:
http://www.journals.uchicago.edu/cgi-bin/contents?ApJL+v647n1

HST Proper Motions and Stellar Dynamics in the Core of 47 Tuc

Preprint (astro-ph/0607597)

Authors: Dean E. McLaughlin, Jay Anderson, Georges Meylan, Karl Gebhardt, Carlton Pryor, Dante Minniti, Sterl Phinney
“HST Proper Motions and Stellar Dynamics in the Core of the Globular Cluster 47 Tucanae”

We have used HST imaging of the central regions (R<100 arcsec, about 5 core radii) of the globular cluster 47 Tucanae to derive proper motions and U- and V-band magnitudes for 14,366 cluster members. We also present a catalogue of astrometry and F475W photometry for nearly 130,000 stars in a rather larger central area. These data are made available in their entirety, in the form of downloadable electronic tables. We use them first to obtain a new estimate for the position of the cluster center and to define the stellar density profile into essentially zero radius. We then search in particular for any very fast-moving stars, such as might be expected to result from very close stellar encounters. Likely fewer than 0.1% (and no more than about 0.3%) of stars have total speeds above the nominal central escape velocity in 47 Tuc, and at lower speeds the velocity distribution is described very well by a regular King model. Considerations of only the proper-motion velocity dispersion then lead to a number of results: (1) Blue stragglers in the core of 47 Tuc have a velocity dispersion lower than that of the cluster giants by a factor of sqrt{2}. (2) The velocity distribution in the cluster center is essentially isotropic, as expected. (3) Using a sample of radial velocities for stars in the core, we estimate the distance to 47 Tuc: D = 4.0 +/- 0.35 kpc. And (4) we infer a 1-sigma upper limit of M<1000-1500 solar masses for any central, intermediate-mass black hole. We can neither confirm nor refute the hypothesis that 47 Tuc might lie on an extension of the M-sigma relation observed for supermassive black holes in galaxy bulges. [Abridged]

http://babbage.sissa.it/abs/astro-ph/0607597

A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in 47 Tuc

New article on ApJ:

Heinke, Craig O.; Rybicki, George B.; Narayan, Ramesh; Grindlay, Jonathan E.
“A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae”

Current X-ray missions are providing high-quality X-ray spectra from neutron stars (NSs) in quiescent low-mass X-ray binaries (qLMXBs). This has motivated us to calculate new hydrogen atmosphere models, including opacity due to free-free absorption and Thomson scattering, thermal electron conduction, and self-irradiation by photons from the compact object. We have constructed a self-consistent grid of neutron star models covering a wide range of surface gravities, as well as effective temperatures, which we make available to the scientific community. We present multiepoch Chandra X-ray observations of the qLMXB X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on timescales from minutes to years. Its high-quality X-ray spectrum is adequately fitted by our hydrogen atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 M_solar is assumed, our spectral fits require that its radius be in the range R_ns=14.5^+1.8_-1.6 km (90% confidence), which is larger than that expected from currently preferred models of NS interiors. If its radius is assumed to be 10 km, then a mass of M_ns=2.20^+0.03_-0.16 M_solar is required. Using models with the appropriate surface gravity for each value of the mass and radius becomes important for interpretation of the highest quality data.

ADS 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