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
Hubble image of NGC 1851
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, if combined with 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] The tidal tail is still present though.[11] 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.[12] The TRAPUM Large Survey Project using the MeerKAT radio telescope discovered thirteen new pulsars in the cluster, which consist of six isolated millisecond pulsars (MSPs) and seven binary pulsars, of which six are MSPs and one is mildly recycled. [13] 43 RR Lyrae variables have been discovered in the cluster, which show this to be an 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.[14] Spectroscopic analysis of the red giant branch member stars suggests there are actually three different populations of stars in the cluster.[15]

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, S2CID 119183070.
  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, S2CID 118649860.
  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, S2CID 17571531.
  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. Carballo-Bello, Julio A.; Martínez-Delgado, David; Navarrete, Camila; Catelan, Márcio; Muñoz, Ricardo R.; Antoja, Teresa; Sollima, Antonio (2017), "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: 683–695, arXiv:1710.08927, doi:10.1093/mnras/stx2767
  12. 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, PMC 6859652, PMID 31762495.
  13. Ridolfi, A.; Freire, P. C. C.; Gautam, T.; Ransom, S. M.; Barr, E. D.; Buchner, S.; Burgay, M.; Abbate, F.; Venkatraman Krishnan, V.; Vleeschower, L.; Possenti, A.; Stappers, B. W.; Kramer, M.; Chen, W.; Padmanabh, P. V.; Champion, D. J.; Bailes, M.; Levin, L.; Keane, E. F.; Breton, R. P.; Bezuidenhout, M.; Grießmeier, J.-M.; Künkel, L.; Men, Y.; Camilo, F.; Geyer, M.; Hugo, B. V.; Jameson, A.; Parthasarathy, A.; Serylak, M. (2022), "TRAPUM discovery of 13 new pulsars in NGC 1851 using MeerKAT", Astronomy & Astrophysics, 664: A27, arXiv:2203.12302, doi:10.1051/0004-6361/202143006, S2CID 248208657
  14. 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): 233, arXiv:1710.03730, Bibcode:2017AJ....154..233S, doi:10.3847/1538-3881/aa94c3, S2CID 119207523, 233.
  15. 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, S2CID 119248182, 19.
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