Messier 67

Messier 67 (also known as M67 or NGC 2682) is an open cluster in the constellation of Cancer. It was discovered by Johann Gottfried Koehler in 1779. Age estimates for the cluster range between 3.2 and 5 billion years. Distance estimates are likewise varied and typically range between 800–900 pc.[1][2][3][4] Estimates of 855, 840, and 815 pc were established via binary star modelling and infrared color-magnitude diagram fitting, accordingly.[2][3][4]

Hertzsprung-Russell diagram for two open clusters, M67 and NGC 188, showing color-magnitude data for two of the best-studied old open clusters.
Messier 67
Messier 67 (SDSS, optical and near-infrared)
Observation data (J2000.0 epoch)
ConstellationCancer
Right ascension08h 51.3m
Declination+11° 49′
Distance~2.61–2.93 kly (800–900 pc[1][2][3][4])
Apparent magnitude (V)6.1
Apparent dimensions (V)30.0′
Physical characteristics
Radius10 ly
Estimated age3.2 to 5 billion years
Other designationsNGC 2682, Cr 204

M67 is not the oldest known open cluster, but Galactic clusters known to be older are few, and none of those is closer than M67. It is an important laboratory for studying stellar evolution, since the cluster is well populated, has negligible amounts of dust obscuration, and all its stars are at the same distance and age, except for approximately 30 anomalous blue stragglers.[5] These blue stragglers are generally the results of mass transfer in binaries or stellar collisions.

M67 is one of the most-studied open clusters, yet estimates of its physical parameters such as age, mass, and number of stars of a given type, vary substantially. Richer et al. estimate its age to be 4 billion years, its mass to be 1080 solar masses, and the number of white dwarfs to be 150.[6] Hurley et al. estimate its current mass to be 1400 solar masses and its initial mass to be approximately 10 times as great.[7]

M67 has more than 100 stars similar to the Sun, and numerous red giants. The total star count has been estimated at well over 500.[8] The ages and prevalence of Sun-like stars contained within the cluster had led astronomers to consider M67 as the possible parent cluster of the Sun.[9] However, computer simulations have suggested that this is highly unlikely to be the case.[10]

The cluster contains no main sequence stars bluer than spectral type F, other than perhaps some of the blue stragglers, since the brighter stars of that age have already left the main sequence. In fact, when the stars of the cluster are plotted on the Hertzsprung-Russell diagram, there is a distinct "turn-off" representing the stars which have terminated hydrogen fusion in the core and are destined to become red giants. As the cluster ages, the turn-off moves progressively down the main sequence.

It appears that M67 does not contain an unbiased sample of stars. One cause of this is mass segregation, the process by which lighter stars gain speed at the expense of more massive stars during close encounters, which causes the lighter stars to be at a greater average distance from the center of the cluster or to escape altogether[11]

A March 2016 joint AIP/JHU study by Barnes at al. on rotational periods of 20 Sun-like stars, measured by the effects of moving starspots on light curves, suggests that these approximately 4 Gyr old stars spin for about 26 days - much like our Sun, which has a period at the equator of 25.38 days.[12] Measurements were carried out as part of the extended K2 mission of Kepler space telescope. This discovery strengthens the solar-stellar connection, a fundamental principle of modern solar and Stellar astrophysics.[13][13]

Artist's impression of a hot Jupiter exoplanet in the star cluster Messier 67.[14]

References
  1. Paunzen, E., Mermilliod, J.-C. "WEBDA: M67"
  2. Sarajedini, Ata; Dotter, Aaron; Kirkpatrick, Allison "Deep 2MASS Photometry of M67 and Calibration of the Main-Sequence J – KS Color Difference as an Age Indicator", AJ, 2009
  3. Yakut, K.; Zima, W.; Kalomeni, B.; van Winckel, H.; Waelkens, C.; De Cat, P.; Bauwens, E.; Vučković, M.; Saesen, S.; Le Guillou, L.; Parmaksızoğlu, M.; Uluç, K.; Khamitov, I.; Raskin, G.; Aerts, C."Close binary and other variable stars in the solar-age Galactic open cluster M 67", A&A, 2009
  4. Majaess, D.; Turner, D.; Lane, D.; Krajci, T. "Deep Infrared ZAMS Fits to Benchmark Open Clusters Hosting delta Scuti Stars", Journal of the American Association of Variable Star Observers, 2011
  5. Xiao-Bin Zhang; Rong-Xian Zhang & Zhi-Ping Li (2005). "S1280 and S1284: Two Oscillating Blue Stragglers in the Open Cluster M67". Chinese Journal of Astronomy and Astrophysics. 5 (6): 579–586. Bibcode:2005ChJAA...5..579Z. doi:10.1088/1009-9271/5/6/003.
  6. Harvey B. Richer; Gregory G. Fahlman; Joanne Rosvick; Rodrigo Ibata (1998). "The White Dwarf Cooling Age of M67". The Astrophysical Journal. 504 (2): L91. arXiv:astro-ph/9806172. Bibcode:1998ApJ...504L..91R. doi:10.1086/311586.
  7. Jarrod R. Hurley; Onno R. Pols; Sverre J. Aarseth; Christopher A. Tout (2005). "A Complete N-body Model of the Old Open Cluster M67". Monthly Notices of the Royal Astronomical Society. 363 (1): 293–314. arXiv:astro-ph/0507239. Bibcode:2005MNRAS.363..293H. doi:10.1111/j.1365-2966.2005.09448.x.
  8. W. L. Sanders (1977). "Membership of the open cluster M67". Astronomy & Astrophysics Supplement Series. 27: 89–116. Bibcode:1977A&AS...27...89S.
  9. "Did Our Solar System Originate in a Distant Star Cluster?". Daily Galaxy. 30 September 2014. Archived from the original on 10 May 2016. Retrieved 30 March 2016.
  10. Pichardo, Bárbara; Moreno, Edmundo; Allen, Christine; Bedin, Luigi R.; Bellini, Andrea; Pasquini, Luca, Christine; Bedin, Luigi R.; Bellini, Andrea; Pasquini, Luca (February 2012). "The Sun was not born in M 67". The Astronomical Journal. 143 (3): 73. arXiv:1201.0987. Bibcode:2012AJ....143...73P. doi:10.1088/0004-6256/143/3/73. article ID 73.CS1 maint: multiple names: authors list (link)
  11. Ch. Bonatto & E. Bica (2003). "Mass segregation in M67 with 2MASS" (PDF). Astronomy and Astrophysics. 405 (2): 525. Bibcode:2003A&A...405..525B. doi:10.1051/0004-6361:20030205.
  12. Sydney A. Barnes; Jörg Weingrill; Dario Fritzewski; Klaus G. Strassmeier; Imants Platais (2016). "Rotation periods for cool stars in the 4 Gyr-old open cluster M67, the solar-stellar connection, and the applicability of gyrochronology to at least solar age". The Astrophysical Journal. 823 (1): 16. arXiv:1603.09179. Bibcode:2016ApJ...823...16B. doi:10.3847/0004-637X/823/1/16.
  13. "Stars nearly as old as Sun found to have similar spin rates". Astronomy Now. 17 May 2016.
  14. "Unexpected Excess of Giant Planets in Star Cluster". Retrieved 22 June 2016.


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