Messier 62

Messier 62 or M62, also known as NGC 6266 or the Flickering Globular Cluster, is a globular cluster of stars in the south^[a] of the equatorial constellation of Ophiuchus. It was discovered in 1771 by Charles Messier,^[b] then added to his catalogue eight years later.^[11]

Messier 62
Messier 62 by the Hubble Space Telescope
Observation data (J2000 epoch)
Class	IV[1]
Constellation	Ophiuchus
Right ascension	17h 01m 12.60s[2]
Declination	–30° 06′ 44.5″[2]
Distance	21.5 ± 1.3 kly (6.6 ± 0.4 kpc)[3]
Apparent magnitude (V)	6.5[4]
Apparent dimensions (V)	15′
Physical characteristics
Absolute magnitude	−9.18.[5]
Mass	1.22×106[6] M☉
Radius	49 ly[7]
Tidal radius	59 ly.[8]
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.02[9] dex
Estimated age	11.78 Gyr[9]
Other designations	C 1658-300, GCl 51, M62, NGC 6266[10]
See also: Globular cluster, List of globular clusters

M62 is about 21.5 kly^[3] from Earth and 5.5 kly from the Galactic Center.^[2] It is among the ten most massive and luminous globular clusters in the Milky Way, showing an integrated absolute magnitude of −9.18.^[5] It has an estimated mass of 1.22×10⁶ M_☉^[6] and a mass-to-light ratio of 2.05±0.04 in the core visible light band, the V band.^[12] It has a projected ellipticity of 0.01, meaning it is essentially spherical.^[13] The density profile of its member stars suggests it has not yet undergone core collapse.^[14] It has a core radius of 1.3 ly (0.39 pc), a half-mass radius of 9.6 ly (2.95 pc), and a half-light radius of 6.0 ly (1.83 pc). The stellar density at the core is 5.13 M_☉ per cubic parsec.^[15] It has a tidal radius of 59 ly (18.0 pc).^[8]

The cluster shows at least two distinct populations of stars, which most likely represent two separate episodes of star formation. Of the main sequence stars in the cluster, 79%±1% are from the first generation and 21%±1% from the second. The second is polluted by materials released by the first. In particular, abundances of helium, carbon, magnesium, aluminium, and sodium differ between these two.^[5]

Indications are this is an Oosterhoff type I, or "metal-rich" system. A 2010 study identified 245 variable stars in the cluster's field, of which 209 are RR Lyrae variables, four are Type II Cepheids, 25 are long period variables, and one is an eclipsing binary. The cluster may prove to be the galaxy's richest in terms of RR Lyrae variables.^[16] It has ten binary millisecond pulsars, including one (M62B) that is displaying eclipsing behavior from gas streaming off its companion,^[17] and one (M62H) with a planetary-mass companion about three times the mass of Jupiter.^[18] There are multiple X-ray sources, including 50 within the half-mass radius.^[14] 47 blue straggler candidates have been identified, formed from the merger of two stars in a binary system, and these are preferentially concentrated near the core region.^[14]

It is hypothesized that this cluster may be host to an intermediate mass black hole (IMBH) – it is considered well-suited for searching for such an object. A brief study, before 2013, of the proper motion of stars within 17″ of the core did not require an IMBH to explain. However, simulations can not rule out one with a mass of a few thousand M_☉ in M62's core. For example, based upon radial velocity measurements within an arcsecond of the core, Kiselev et al. (2008) made the claim of an IMBH in M15, likewise with mass of (1–9)×10³ M_☉.^[12]