47 Tucanae

47 Tucanae

After Omega Centauri, 47 Tucanae is the brightest globular cluster in the night sky.[1]
Observation data (J2000 epoch)
Class III[2]
Constellation Tucana
Right ascension 00h 24m 05.67s[3]
Declination –72° 04′ 52.6″[3]
Distance 4.0 ± 0.35 kpc (13,000 ± 1,100 ly)[4]
Apparent magnitude (V) +4.09[3]
Apparent dimensions (V) 30′.9
Physical characteristics
Mass 7.00×105[5] M☉
Radius 60 ly[6]
VHB 14.2
Metallicity  = –0.78[7] dex
Estimated age 13.06 Gyr[7]
Notable features 2nd brightest globular cluster after Omega Centauri
Other designations ξ Tuc, NGC 104, GCl 1, Mel 1 [3] Caldwell 106 1RXS J002404.6-720456

47 Tucanae, 47 Tuc (or NGC 104) is a globular cluster located in the constellation Tucana. It is about 4.0 ± 0.35 kpc (13,000 ± 1,100 ly) away from Earth,[8] and 120 light years across.[9] 47 Tuc can be seen with the naked eye, with the apparent magnitude of 4.1v,[3] and whose apparent size subtends about 50′ or 0.9° across. Due to its far southern location just 18° from the south celestial pole, it was not discovered until the 1750s, when the cluster was first identified by Nicolas-Louis de Lacaille from South Africa.

47 Tucanae is the second brightest globular cluster (following Omega Centauri), and telescopically reveals about ten thousand stars, many appearing within a small dense central core. In February 2017, indirect evidence for a likely intermediate-mass black hole in 47 Tucanae was announced.[10]

Early history

It was first given its Bayer designation ξ Tucanae by Johann Bayer in 1603 within the published Uranometria Omnium Asterismorum ("Uranometry of all the asterisms"), or now simply as "Uranometria" , but was discovered as a cluster by Nicolas-Louis de Lacaille in 1751-2, who initially thought it was the nucleus of a bright comet.[11] It then become the first object listed in his deep-sky catalogue as 'Lac I-1'. Its designated number '47' was later assigned in "Allgemeine Beschreibung und Nachweisung der Gestirne nebst Verzeichniss" (General description and verification of the stars and indexes) compiled by Johann Elert Bode and was published in Berlin during 1801. Bode did not observe this cluster himself, but had reordered Lacaille's catalogued stars by constellation by order of Right Ascension.

Characteristics

47 Tucanae is the second brightest globular cluster in the sky (after Omega Centauri), and is noted for having a small very bright and dense core. It is one of the most massive globular clusters in the Galaxy, containing millions of stars. The cluster appears roughly the size of the full moon in the sky under ideal conditions. Though it appears adjacent to the Small Magellanic Cloud, the latter is some 60.6 ± 1.0 kpc (200,000 ± 3,300 ly) distant,[12] being over fifteen times farther than 47 Tuc.

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 ten to fifteen were expected based on the rate of planet discoveries around stars near the Sun. This indicates that planets are relatively rare in globular clusters.[13] 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.[14] 47 Tucanae contains at least 21 blue stragglers near its core.[15] It also contains hundreds of X-ray sources, including stars with enhanced chromospheric activity due to their presence in binary star systems, cataclysmic variable stars 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.[16] 47 Tucanae has 25 known[17] millisecond pulsars, the second largest population of pulsars in any globular cluster.[18] 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.[19] X-ray emission has been individually detected from most millisecond pulsars in 47 Tucanae with the Chandra X-ray Observatory, likely emission from the neutron star surface,[20] and gamma-ray emission has been detected with the Fermi Gamma-ray Space Telescope from its millisecond pulsar population (making 47 Tucanae the first globular cluster to be detected in gamma-rays).[21]

It is not yet clear whether 47 Tucanae hosts a central black hole. Hubble Space Telescope data constrain the mass of any possible black hole at the cluster's center to be less than approximately 1,500 solar masses.[8] However, in February, 2017, astronomers announced that a black hole of some 2,200 solar masses may be located in the cluster; the researchers detected the black hole's signature from the motions and distributions of pulsars in the cluster.[10]

Modern Discoveries

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.[22]

In May 2015, the first evidence of the process of mass segregation in this globular cluster were announced.[23]

Then in February 2017, the discovery was announced of the first known intermediate-mass black hole, about 2,200 times the mass of the Sun. Within the globular cluster, it was found by the proper motions of its surrounding stars.[10]

References

  1. ↑ "Retirement in the suburbs". Retrieved 12 June 2015.
  2. ↑ Shapley, Harlow; Sawyer, Helen B. (August 1927). A Classification of Globular Clusters. Harvard College Observatory Bulletin. pp. 11–14. Bibcode:1927BHarO.849...11S.
  3. 1 2 3 4 5 "SIMBAD Astronomical Database". Results for NGC 104. Retrieved 2006-11-17.
  4. ↑ Carretta, E.; et al. "Distances, Ages, and Epoch of Formation of Globular Clusters". The Astrophysical Journal. Bibcode:2000ApJ...533..215C. arXiv:astro-ph/9902086 Freely accessible. doi:10.1086/308629. Retrieved 2008-11-12.
  5. ↑ Marks, Michael; Kroupa, Pavel (August 2010), "Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way", Monthly Notices of the Royal Astronomical Society, 406 (3): 2000–2012, Bibcode:2010MNRAS.406.2000M, arXiv:1004.2255 Freely accessible, doi:10.1111/j.1365-2966.2010.16813.x. Mass is from MPD on Table 1.
  6. ↑ distance × sin( diameter_angle / 2 ) = 60 ly. radius
  7. 1 2 Forbes, Duncan A.; Bridges, Terry (May 2010), "Accreted versus in situ Milky Way globular clusters", Monthly Notices of the Royal Astronomical Society, 404 (3): 1203–1214, Bibcode:2010MNRAS.404.1203F, arXiv:1001.4289 Freely accessible, doi:10.1111/j.1365-2966.2010.16373.x.
  8. 1 2 McLaughlin, D.E.; et al. (2006). "Hubble Space Telescope Proper Motions and Stellar Dynamics in the Core of the Globular Cluster 47 Tucanae". Astrophysical Journal Supplement Series. 166 (1): 249–297. Bibcode:2006ApJS..166..249M. arXiv:astro-ph/0607597 Freely accessible. doi:10.1086/505692.
  9. ↑ "47 Tuc: A Great Globular Cluster of Stars". Astronomy Picture of the Day. August 26, 2008. Retrieved 25 May 2017.
  10. 1 2 3 Kızıltan, Bülent; Baumgardt, Holger; Loeb, Abraham (2017). "An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae". Nature. 542: 203–205. Bibcode:2017arXiv170202149K. doi:10.1038/nature21361.
  11. ↑ O'Meara, Stephen James (2013). Deep-Sky Companions: Southern Gems. Cambridge, United Kingdom: Cambridge University Press. pp. 16–17. ISBN 9781107015012.
  12. ↑ Hilditch, R. W.; Howarth, I. D.; Harries, T. J. (2005). "Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance". Monthly Notices of the Royal Astronomical Society. 357 (1): 304–324. Bibcode:2005MNRAS.357..304H. arXiv:astro-ph/0411672 Freely accessible. doi:10.1111/j.1365-2966.2005.08653.x.
  13. ↑ "A Shortage of Planets". Retrieved 16 November 2010.
  14. ↑ Bryner, Jeanna. "Mass Migration: How Stars Move in Crowd". Retrieved 14 November 2010.
  15. ↑ "NASA's Hubble Space Telescope Finds "Blue Straggler" Stars in the Core of a Globular Cluster". Hubble News Desk. 1991-07-24. Retrieved 5 May 2006.
  16. ↑ 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 47 Tuc". Science. 292 (5525): 2290–2295. Bibcode:2001Sci...292.2290G. PMID 11358997. arXiv:astro-ph/0105528 Freely accessible. doi:10.1126/science.1061135.
  17. ↑ "The 25 millisecond radio pulsars in 47 Tucanae".
  18. ↑ "Pulsars in Globular Clusters".
  19. ↑ 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. Bibcode:2005ApJ...630.1029B. arXiv:astro-ph/0506031 Freely accessible. doi:10.1086/432249.
  20. ↑ 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. Bibcode:2006ApJ...646.1104B. arXiv:astro-ph/0604318 Freely accessible. doi:10.1086/505133.
  21. ↑ Abdo, A. A.; et al. (2009). "Detection of High-Energy Gamma-Ray Emission from the Globular Cluster 47 Tucanae with Fermi". Science. 325 (5942): 845–848. Bibcode:2009Sci...325..845A. PMID 19679807. doi:10.1126/science.1177023.
  22. ↑ "The Australian Optical SETI Project" (PDF).
  23. ↑ "Hubble Catches Stellar Exodus in Action". Space Daily. 18 May 2015.
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