NGC 1851

NGC 1851 (also known as Caldwell 73)[9] is a relatively massive[3] globular cluster located in the southern constellation of Columba. Astronomer John Dreyer described it as not very bright but very large, round, well resolved, and clearly consisting of stars.[4] It is located 39.5 kilolight-years from the Sun, and 54.1 kilolight-years from the Galactic Center.[3] The cluster is following a highly eccentric orbit through the galaxy, with an eccentricity of about 0.7[10]

NGC 1851
An Ultraviolet image of NGC 1851.
Credit: NASA/GALEX
Observation data (J2000 epoch)
ClassII[1]
ConstellationColumba
Right ascension05h 14m 06.76s[2]
Declination–40° 02′ 47.6″[2]
Distance39.5 kly (12.1 kpc)[3]
Apparent magnitude (V)7.3[4]
Apparent dimensions (V)11′[4]
Physical characteristics
Absolute magnitude−7.80[3]
Mass5.51×105[5] M☉
Metallicity = −1.27[3] dex
Estimated age9.2 Gyr[6]
Other designationsGCl 9,[7] ESO 305-SC 016,[8] Caldwell 73, Melotte 30

This object has a Shapley–Sawyer Concentration Class of II,[1] indicating a dense central concentration. It has one of the highest concentrations known for Galactic globular clusters.[3] The stellar components shows two separate populations of subgiant stars, with the brighter branch being more concentrated in the outer regions of the cluster.[10] NGC 1851 is an estimated 9.2[6] billion years old with 551,000 times the mass of the Sun.[5]

The cluster is surrounded by a diffuse halo of stars that stretches outward to a radius of 240 pc or more. This feature, and the lack of tidal tail or associated stream of stars, suggests the cluster may be a stripped dwarf galaxy nuclei, similar to Omega Centauri, that has been accreted by the Milky Way.[3] It is also possible the cluster is the result of the merger of two separate clusters, but the fact that they would need to have the same metallicity – what astronomers term the abundance of elements other than hydrogen and helium – makes this scenario less likely.[10]

PSR J0514-4002A is a milli-second pulsar in NGC 1851. It is orbiting a massive object that may also be a neutron star. The pair have an orbital period of 18.8 days with a large eccentricity of 0.89.[11] 43 RR Lyrae variables have been discovered in the cluster, which show this to be Oosterhoff type I cluster but having properties similar to type II. Two populations of horizontal branch stars have been observed, with the pair having an age difference of around two billion years.[12] Spectroscopic analysis of the red giant branch member stars suggests there are actually three different populations of stars in the cluster.[13]

References
  1. Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
  2. Goldsbury, Ryan; et al. (December 2010), "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters", The Astronomical Journal, 140 (6): 1830–1837, arXiv:1008.2755, Bibcode:2010AJ....140.1830G, doi:10.1088/0004-6256/140/6/1830.
  3. Kuzma, P. B.; et al. (January 2018), "The outer envelopes of globular clusters. II. NGC 1851, NGC 5824 and NGC 1261*", Monthly Notices of the Royal Astronomical Society, 473 (3): 2881–2898, arXiv:1709.02915, Bibcode:2018MNRAS.473.2881K, doi:10.1093/mnras/stx2353.
  4. "SEDS NGC Catalog Online". Results for NGC 1851. Retrieved 2011-01-09.
  5. Boyles, J.; et al. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal, 742 (1): 51, arXiv:1108.4402, Bibcode:2011ApJ...742...51B, doi:10.1088/0004-637X/742/1/51.
  6. Koleva, M.; et al. (April 2008), "Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting", Monthly Notices of the Royal Astronomical Society, 385 (4): 1998–2010, arXiv:0801.0871, Bibcode:2008MNRAS.385.1998K, doi:10.1111/j.1365-2966.2008.12908.x.
  7. "NGC 1851". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2020-04-10.
  8. "NASA/IPAC Extragalactic Database". Results for NGC 1851. Retrieved 2011-01-09.
  9. O'Meara, Stephen James (2016), Deep-Sky Companions: The Caldwell Objects, Cambridge University Press, p. 344, ISBN 9781107083974.
  10. Carballo-Bello, Julio A.; et al. (February 2018), "Tails and streams around the Galactic globular clusters NGC 1851, NGC 1904, NGC 2298 and NGC 2808", Monthly Notices of the Royal Astronomical Society, 474 (1): 683–695, arXiv:1710.08927, Bibcode:2018MNRAS.474..683C, doi:10.1093/mnras/stx2767.
  11. Ridolfi, A.; et al. (December 2019), "Upgraded Giant Metrewave Radio Telescope timing of NGC 1851A: a possible millisecond pulsar - neutron star system", Monthly Notices of the Royal Astronomical Society, 490 (3): 3860–3874, arXiv:1909.06163, Bibcode:2019MNRAS.490.3860R, doi:10.1093/mnras/stz2645.
  12. Subramaniam, Annapurni; et al. (December 2017), "The Horizontal Branch Population of NGC 1851 as Revealed by the Ultraviolet Imaging Telescope (UVIT)", The Astronomical Journal, 154 (6), arXiv:1710.03730, Bibcode:2017AJ....154..233S, doi:10.3847/1538-3881/aa94c3, 233.
  13. Lim, Dongwook; et al. (January 2015), "Low-resolution Spectroscopy for the Globular Clusters with Signs of Supernova Enrichment: M22, NGC 1851, and NGC 288", The Astrophysical Journal Supplement, 216 (1): 13, arXiv:1412.1832, Bibcode:2015ApJS..216...19L, doi:10.1088/0067-0049/216/1/19, 19.

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