Terzan 5

Terzan 5 is a heavily obscured globular cluster belonging to the bulge (the central star concentration) of the Milky Way galaxy.[3] It was one of six globulars discovered by French[7] astronomer Agop Terzan in 1968[8] and was initially labeled Terzan 11. The cluster was cataloged by the Two-Micron Sky Survey as IRC–20385.[9] It is situated in the Sagittarius constellation in the direction of the Milky Way's center. Terzan 5 probably follows an unknown complicated orbit around the center of the galaxy, but currently it is moving towards the Sun with a speed of around 90 km/s.[6]

Terzan 5
Hubble image of Terzan 5
Credit: NASA/ESA/Hubble/F. Ferraro
Observation data (J2000 epoch)
ClassG2[1]
ConstellationSagittarius
Right ascension17h 48m 05s[2]
Declination−24° 46′ 48″[2]
Distance18.8 ± 1.6 kly[note 1] (5.9 ± 0.5 kpc[4])
Apparent magnitude (V)12.8[1]
Apparent dimensions (V)1′02″ (half-mass diameter)[4]
Physical characteristics
Mass~2×106[4] M☉ (4 × 1036 kg)
Radius2.7 ly[note 2]
VHB22.5[3]
Metallicity = −0.21 dex
Estimated age12 Gyr[6]
Notable featuresPossibly the core of a disrupted dwarf galaxy
Other designationsTer 5, IRC–20385

Physical properties

The absolute magnitude of Terzan 5 is at least MV=−7.5.[1] Its bolometric luminosity is about 800,000 times that of the Sun, while its mass is about 2 million solar masses.[4] The small core of Terzan 5—about 0.5 pc in size[4]—has one of the highest star densities in the galaxy. Its volume mass density exceeds 106 M☉/pc3,[10] while its volume luminosity density exceeds 105.5 L☉/pc3, where M☉ and L☉ are the Sun's mass and luminosity, respectively. The cluster also has one of the highest metallicities among the Milky Way's globular clusters—[Fe/H]=−0.21.[5]

In 2009 it was discovered that Terzan 5 consists of at least two generations of stars with ages of 12 and 4.5 billion years and slightly different metallicities, possibly indicating that it is the core of a disrupted dwarf galaxy, not a true globular cluster.[6] There are only a few other globular clusters in the Milky Way that contain stars with different ages. Among them are M54 and Omega Centauri. The cluster also contains around 1300 core helium burning horizontal branch (HB) stars,[6] including at least one RR Lyrae variable star.[10]

Pulsars and X-ray sources

Terzan 5 is known to contain at least 34 millisecond radio pulsars;[11] their true number may be as high as 200.[12] The first such object, PSR B1744-24A, discovered in 1990, has the period of 11.56 ms. The population of pulsars inside Terzan 5 includes PSR J1748–2446ad, the fastest known millisecond pulsar, which is spinning at 716 Hz (the rotation period is 1.40 ms).[11]

Terzan 5 also contains an X-ray burster, discovered in 1980, known as Terzan 5 or XB 1745-25.[13] It also contains around 50 weaker X-ray sources, many of which are likely Low-mass X-ray binaries (LMXB) or cataclysmic variables.[14]

The large number of X-ray sources and millisecond pulsars may be a direct consequence of the high density of the cluster's core, which leads to a high rate of star collisions, and to formation of close binaries, including binary systems which contain a neutron star.[14]

In addition to discrete X-ray sources Terzan 5 produces a diffuse non-thermal X-ray emission and high (a few GeV) and ultra-high (0.5–24 TeV) energy gamma-rays. The high energy gamma rays probably originate in the magnetosphere of abundant millisecond pulsars, while ultra-high energy gamma rays likely result from the inverse Compton scattering by the relativistic electron emitted by the pulsars off the cosmic microwave background radiation.[12]

  • Hubble image of the ancient globular cluster Terzan 5
  • Terzan 5 is one of the bulge's primordial building blocks, most likely the relic of the very early days of the Milky Way.[15]

Notes
  1. The distance estimates have historically varied from as close as 2.3 kpc[1] to as far as 14.6 kpc.[3] The recent estimates generally range from 5.5 to 8.7 kpc.[4][5]
  2. Based on the half-mass radius of 31″[4] and the known distance of 5.9 kpc. The dense core of the cluster has the radius of 9.0″.[4]

References
  1. Racine, René (1975). "UBV photometry of faint globular clusters". The Astronomical Journal. 80 (12): 1031–36. Bibcode:1975AJ.....80.1031R. doi:10.1086/111835.
  2. "Cl Terzan 5". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2009-12-04.
  3. Ortolani, S.; Barbuy, B.; Bica, E. (1996). "NTT VI photometry of the metal rich and obscured bulge globular cluster Terzan 5". Astronomy and Astrophysics. 308: 733–37. Bibcode:1996A&A...308..733O.
  4. Lanzoni, B.; Ferraro, F. R.; Dalessandro, E.; Mucciarelli, A.; Beccari, G.; Miocchi, P.; Bellazzini, M.; Rich, R. M.; Origlia, L.; Valenti, E.; Rood, R. T.; Ransom, S. M. (2010). "New Density Profile and Structural Parameters of the Complex Stellar System Terzan 5". The Astrophysical Journal. 717 (2): 653. arXiv:1005.2847. Bibcode:2010ApJ...717..653L. doi:10.1088/0004-637X/717/2/653.
  5. Ortolani, S.; Barbuy, B.; Zoccali, M.; Renzini, A.; Renzini, A. (2007). "Distances of the bulge globular clusters Terzan 5, Liller 1, UKS 1 and Terzan 4 based on HST NICMOS photometry". Astronomy and Astrophysics. 470 (3): 1043–49. arXiv:0705.4030. Bibcode:2007A&A...470.1043O. doi:10.1051/0004-6361:20066628.
  6. Ferraro, F.R.; Dalessandro, E.; Mucciarelli, A.; Beccari, G; Rich, RM; Origlia, L; Lanzoni, B; Rood, RT; et al. (2009). "The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge". Nature. 462 (7272): 483–486. arXiv:0912.0192. Bibcode:2009Natur.462..483F. doi:10.1038/nature08581. PMID 19940920.
  7. Gottlieb, Steve (August 1, 2000). Sky and Telescope. 100 (2): 112. ISSN 0037-6604. Missing or empty |title= (help)
  8. Terzan, Agop (1968). "Six nouveaux amas stellaires (Terzan 3-8) dans la region DU centre de la Voie lactee et les constellations DU Scorpion et DU Sagittaire". C. R. Acad. Sci. 267 (Ser. B): 1245–1248. Bibcode:1968CRASB.267.1245T.
  9. King, I.R. (1972). "The identity and aliases of the stellar system Terzan 5". Astronomy and Astrophysics. 19: 166. Bibcode:1972A&A....19..166K.
  10. Cohn, Haldan N.; Lugger, Phyllis M.; Grindlay, Jonathan E.; Edmonds, Peter D. (2002). "Hubble Space Telescope/NICMOS observations of Terzan 5: stellar content and structure of the core". The Astrophysical Journal. 571 (2): 818–29. arXiv:astro-ph/0202059. Bibcode:2002ApJ...571..818C. doi:10.1086/339874.
  11. Hessels, J. W. T.; Ransom, S. M.; Stairs, I. H.; Freire, P. C.; Kaspi, V. M.; Camilo, F. (2006). "A Radio Pulsar Spinning at 716 Hz". Science. 311 (5769): 1901–1904. arXiv:astro-ph/0601337. Bibcode:2006Sci...311.1901H. doi:10.1126/science.1123430. PMID 16410486.
  12. Abramowski, A.; Acero, F.; Aharonian, F.; Akhperjanian, A. G.; Anton, G.; Balzer, A.; Barnacka, A.; Barres De Almeida, U.; Becherini, Y.; Becker, J.; Behera, B.; Bernlöhr, K.; Bochow, A.; Boisson, C.; Bolmont, J.; Bordas, P.; Brucker, J.; Brun, F.; Brun, P.; Bulik, T.; Büsching, I.; Carrigan, S.; Casanova, S.; Cerruti, M.; Chadwick, P. M.; Charbonnier, A.; Chaves, R. C. G.; Cheesebrough, A.; Chounet, L. -M.; Clapson, A. C. (2011). "Very-high-energy gamma-ray emission from the direction of the Galactic globular cluster Terzan 5". Astronomy & Astrophysics. 531: L18. arXiv:1106.4069. Bibcode:2011A&A...531L..18H. doi:10.1051/0004-6361/201117171.
  13. Makishima, K.; Ohashi, T.; Inoue, H.; Koyama, K.; Matsuoka, M.; Murakami, T.; Oda, M.; Ogawara, Y.; Shibazaki, N.; Tanaka, Y.; Kondo, I.; Hayakawa, S.; Kunieda, H.; Makino, F.; Masai, K.; Nagase, F.; Tawara, Y.; Miyamoto, S.; Tsunemi, H.; Da Yamashita, K. (1981). "Discovery of two new burst sources in the globular clusters Terzan 1 and Terzan 5". The Astrophysical Journal. 247: L23–25. Bibcode:1981ApJ...247L..23M. doi:10.1086/183581.
  14. Heinke, C.O.; Wijnands, R.; Cohn, H.N.; Lugger, P. M.; Grindlay, J. E.; Pooley, D.; Lewin, W. H. G. (2006). "Faint X-ray sources in the globular cluster Terzan 5". The Astrophysical Journal. 651 (2): 1098–1111. arXiv:astro-ph/0606253. Bibcode:2006ApJ...651.1098H. doi:10.1086/507884.
  15. "The unusual cluster Terzan 5". www.eso.org. Retrieved 9 September 2016.
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