47 Tucanae

47 Tucanae
47 Tucanae—Image mosaic by E. Kopan (IPAC) (Field size 19.2´ × 23.3´)
Observation data (J2000 epoch)
Class	III
Constellation	Tucana
Right ascension	00h 24m 05.67s[1]
Declination	-72° 04′ 52.6″[1]
Distance	16,700 ± 850 light years (5130 ± 250 parsecs[2])
Apparent magnitude (V)	+4.91[1]
Apparent dimensions (V)	30′.9
Physical characteristics
Mass	106 [3] M☉
Radius	60 ± ? ly[4]
VHB	14.2
Estimated age	~ 10 Billion Years
Notable features	2nd brightest globular cluster after Omega Centauri
Other designations	ξ Tuc, NGC 104, GCl 1[1]
See also: Globular cluster, List of globular clusters

47 Tucanae (NGC 104) or just 47 Tuc is a globular cluster located in the constellation Tucana. It is about 16,700 light years away from Earth, and 120 light years across. It can be seen with the naked eye, with a visual magnitude of 4.0. Its number comes not from the Flamsteed catalogue, but the more obscure 1801 "Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss" compiled by Johann Elert Bode. 47 Tucanae is included in Sir Patrick Moore's Caldwell catalogue as C106.

47 Tucanae was discovered by Nicolas Louis de Lacaille in 1751, its southern location having hidden it from European observers until then. The cluster appears roughly the size of the full moon in the sky under ideal conditions. It is the second brightest globular cluster in the sky (after Omega Centauri), and is noted for having a very bright and dense core. It is also one of the most massive globular clusters in the Galaxy, containing millions of stars.

The core of 47 Tuc was the subject of a major survey for planets, using the Hubble Space Telescope to look for partial eclipses of stars by their planets. No planets were found, though 10-15 were expected based on the rate of planet discoveries around stars near the Sun. This indicates that planets are relatively rare in globular clusters.^[5] A later ground-based survey in the uncrowded outer regions of the cluster also failed to detect planets when several were expected. This strongly indicates that the low metallicity of the environment, rather than the crowding, is responsible.

47 Tuc's dense core contains a number of exotic stars of scientific interest. Globular clusters efficiently sort stars by mass, with the most massive stars falling to the center.^[6] 47 Tuc contains at least 21 blue stragglers near its core.^[7] It also contains hundreds of X-ray sources, including stars with enhanced chromospheric activity due to their presence in binary star systems, cataclysmic variables containing white dwarfs accreting from companion stars, and low-mass X-ray binaries containing neutron stars that are not currently accreting, but can be observed by the X-rays emitted from the hot surface of the neutron star.^[8] 47 Tuc has 23 known millisecond pulsars, the second largest population of pulsars in any globular cluster.^[9] These pulsars are thought to be spun up by the accretion of material from binary companion stars, in a previous X-ray binary phase. The companion of one pulsar in 47 Tucanae, 47 Tuc W, seems to still be transferring mass towards the neutron star, indicating that this system is completing a transition from being an accreting low-mass X-ray binary to a millisecond pulsar.^[10] X-ray emission has been individually detected from most millisecond pulsars in 47 Tuc with the Chandra X-ray Observatory, likely emission from the neutron star surface,^[11] and gamma-ray emission has been detected with the Fermi Gamma-ray Space Telescope from its millisecond pulsar population (making 47 Tuc the first globular cluster to be detected in gamma-rays).^[12] There is no evidence yet for the existence of any black holes in 47 Tuc; Hubble Space Telescope data provides the strongest constraint on the mass of any possible black hole at its center, < 1500 times the mass of our Sun.^[13]

In December 2008, Ragbir Bhathal of the University of Western Sydney claimed the detection of a strong laser-like signal from the direction of 47 Tucanae.^[14]

Globular Cluster 47 Tuc. Credit ESO

References

1. ^ ^{a b c d} "SIMBAD Astronomical Database". Results for NGC 104. http://simbad.u-strasbg.fr/Simbad. Retrieved 2006-11-17. 
2. ^ "Distances, Ages, and Epoch of Formation of Globular Clusters". Paper in The Astrophysical Journal. http://adsabs.harvard.edu/abs/2000ApJ...533..215C. Retrieved 2008-11-12. 
3. ^ "eso0620 - The Toucan's Diamond". ESO. http://www.eso.org/public/news/eso0620/. 
4. ^ distance × sin( diameter_angle / 2 ) = 60 ly. radius
5. ^ "A Shortage of Planets". http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=192. Retrieved 16 November 2010. 
6. ^ Bryner, Jeanna. "Mass Migration: How Stars Move in Crowd". http://www.space.com/scienceastronomy/061030_mm_star_migration.html. Retrieved 14 November 2010. 
7. ^ "NASA's Hubble Space Telescope Finds "Blue Straggler" Stars in the Core of a Globular Cluster". Hubble News Desk. 1991-07-24. http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/. Retrieved 2006-05-24. 
8. ^ Grindlay, Jonathan E.; Heinke, Craig O.; Edmonds, Peter D.; Murray, Steve S. (2001). "High-Resolution X-ray Imaging of a Globular Cluster Core: Compact Binaries in 47Tuc". Science 292 (5525): 2290–2295. arXiv:astro-ph/0105528. Bibcode 2001Sci...292.2290G. doi:10.1126/science.1061135. PMID 11358997. 
9. ^ "The 23 millisecond radio pulsars in 47 Tucanae". http://www.naic.edu/~pfreire//47Tuc/. 
10. ^ Bogdanov, Slavko; Grindlay, Jonathan E.; van den Berg, Maureen (2005). "An X-Ray Variable Millisecond Pulsar in the Globular Cluster 47 Tucanae: Closing the Link to Low-Mass X-Ray Binaries". Astrophysical Journal 630 (2): 1029–1036. arXiv:astro-ph/0506031. Bibcode 2005ApJ...630.1029B. doi:10.1086/432249. 
11. ^ Bogdanov, Slavko; Grindlay, Jonathan E.; Heinke, Craig O.; Camilo, Fernando; Freire, Paulo C. C.; Becker, Werner (2006). "Chandra X-Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae". Astrophysical Journal 646 (2): 1104–1115. arXiv:astro-ph/0604318. Bibcode 2006ApJ...646.1104B. doi:10.1086/505133. 
12. ^ Abdo, AA; Ackermann, M; Ajello, M; Atwood, WB; Axelsson, M; Baldini, L; Ballet, J; Barbiellini, G et al. (2009). "Detection of High-Energy Gamma-Ray Emission from the Globular Cluster 47 Tucanae with Fermi". Science 325 (5942): 845-. Bibcode 2009Sci...325..845A. doi:10.1126/science.1177023. PMID 19679807. 
13. ^ McLaughlin, Dean E.; Anderson, Jay; Meylan, Georges; Gebhardt, Karl; Pryor, Carlton; Minniti, Dante; Phinney, Sterl (2006). "Hubble Space Telescope Proper Motions and Stellar Dynamics in the Core of the Globular Cluster 47 Tucanae". Astrophysical Journal Supplement 166 (1): 249–297. arXiv:astro-ph/0607597. Bibcode 2006ApJS..166..249M. doi:10.1086/505692. 
14. ^ "The Australian Optical SETI Project". http://www.lpi.usra.edu/meetings/abscicon2010/pdf/5046.pdf. 

External links

• 47 Tucanae at the ESA-Hubble website
• 47 Tucanae, Galactic Globular Clusters Database page
• 47 Tucanae at the Chandra X-ray Observatory website