NGC 6388 is aglobular cluster of stars located in the southernconstellation ofScorpius. The cluster was discovered by Scottish astronomerJames Dunlop on May 13, 1826 using a 20 cm (9 in)reflector telescope.[3] It was later determined to be a globular cluster by English astronomerJohn Herschel, who was able to resolve it into individual stars.[3] NGC 6388 is located at a distance of approximately 35,600light-years (10.90 kpc) from the Sun.[5] Due to itsapparent visual magnitude of +6.8,[3] binoculars or a small telescope are required to view it.
This cluster has an age of11.5±1.5 Gyr and acore radius of7.2″. The centralvelocity dispersion is18.9 km/s,[8] and the central density is2.19×105L☉·pc−3.[9] With an estimated mass of2.17×106M☉,[6] it is one of the most massive globular clusters in the Milky Way. Multiple stellar populations have been detected.[8] In the past it has been proposed that this cluster was accreted into the Milky Way galaxy. However, the abundance of stellar elements from a large sample of cluster members strongly indicate it was formed within the galaxy.[10][11]
TheHertzsprung–Russell diagram for NGC 6388 shows that themain-sequence turnoff for this cluster is similar to that of the cluster47 Tucanae. This suggests that the bulk of the stellar components have a similar age and chemical abundances between the two clusters. However, theRR Lyrae variables and hotter members of thehorizontal branch in NGC 6388 appear overluminous.[12] It has been suggested that this extended horizontal branch may be the result of a centralintermediate mass black hole (IMBH). X-ray observations of the cluster rule out an accreting IMBH, but not a quiescent black hole.[8] The stellar density and radial profiles of velocity dispersion rule out an IMBH with a mass greater than about2,000 M☉.[13]
The cluster contains a population ofblue straggler stars (BSS), which are the result of stellar mergers. The radial distribution of this population is high near the core, decreasing at intermediate distance before increasing again at larger radii. This bimodal distribution is similar to that in other globulars. However, the scale of the population at intermediate distances suggests thatdynamic friction in NGC 6388 is less efficient at segregating BSS toward the core.[14]
Examination of NGC 6388 by theChandra observatory show 61X-ray sources within a half-mass radius of the core (0.67′), with all having luminosities above5×1030 erg. Five of these appear to be X-ray binaries.[9]
^Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters".Harvard College Observatory Bulletin.849 (849):11–14.Bibcode:1927BHarO.849...11S.
^"NCG 6388".Students for the Exploration and Development of Space. Retrieved26 September 2015.
^abWatkins, Laura L.; et al. (October 2015). "Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. III. Dynamical Distances and Mass-to-Light Ratios".The Astrophysical Journal.812 (2). id. 149.arXiv:1509.00513.Bibcode:2015ApJ...812..149W.doi:10.1088/0004-637X/812/2/149.
^Lanzoni, B.; et al. (June 2013). "The Velocity Dispersion Profile of NGC 6388 from Resolved-star Spectroscopy: No Evidence of a Central Cusp and New Constraints on the Black Hole Mass".The Astrophysical Journal.769 (2). id. 107.arXiv:1304.2953.Bibcode:2013ApJ...769..107L.doi:10.1088/0004-637X/769/2/107.
^Dalessandro, E.; et al. (2008). "Blue straggler stars in the unusual globular cluster NGC 6388".Memorie della Società Astronomica Italiana.79: 698.Bibcode:2008MmSAI..79..698D.
Dalessandro, E.; et al. (2008). "Blue straggler stars in the unusual globular cluster NGC 6388".Memorie della Società Astronomica Italiana.79: 698.Bibcode:2008MmSAI..79..698D.
Lanzoni, B.; et al. (October 2007). "The Surface Density Profile of NGC 6388: A Good Candidate for Harboring an Intermediate-Mass Black Hole".The Astrophysical Journal.668 (2):L139 –L142.arXiv:0709.0119.Bibcode:2007ApJ...668L.139L.doi:10.1086/522927.
Busso, G.; et al. (October 2007). "The peculiar horizontal branch morphology of the Galactic globular clusters NGC 6388 and NGC 6441: new insights from UV observations".Astronomy and Astrophysics.474 (1):105–119.arXiv:0708.1736.Bibcode:2007A&A...474..105B.doi:10.1051/0004-6361:20077806.
Hughes, Joanne; et al. (July 2007). "Washington Photometry of the Red Giant Branch in NGC 6388".The Astronomical Journal.134 (1):229–240.Bibcode:2007AJ....134..229H.doi:10.1086/518559.
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