Cygnus OB2

Cygnus OB2 is an OB association that is home to some of the most massive and most luminous stars known, including suspected Luminous blue variable Cyg OB2 #12. It also includes one of the largest known stars, NML Cygni.[3] The region is embedded within a wider one of star formation known as Cygnus X, which is one of the most luminous objects in the sky at radio wavelengths. The region is approximately 1,400 parsecs from Earth in the constellation of Cygnus.[2]

Cygnus OB2
Cygnus OB2 in the light of H-Alpha; 2.5° view
Credit: IPHAS
Observation data (J2000.0 epoch)
ConstellationCygnus
Right ascension 20h 33m 12s[1]
Declination41° 19′ 00″
Distance4.7 kly (1400 pc[2])
Apparent magnitude (V)6.1
Apparent dimensions (V)60.0′
Physical characteristics
Estimated age1 to 7 million years
Notable features–

The young cluster is one of the largest known and the largest in the northern hemisphere with some authors formerly classifying it as a young globular cluster similar to those in the Large Magellanic Cloud.[4] Today, however, it is considered a massive, low-density stellar association.[5]

Although it is over ten times more massive than the Orion Nebula, which is easily seen with the naked eye, Cygnus OB2 is hidden behind a massive dust cloud known as the Cygnus Rift, which obscures many of the stars in it. This means that despite its large size, it is hard to determine its actual properties. The estimated number of massive stars range from 50[6] to 100[4] of spectral type O and its total mass having been calculated as (4–10)×104[4] or 3×104 solar masses according to other investigations.[6]

Despite this, recent surveys ranging from radio to X-ray wavelengths have observed the region to great depths to gain a better understanding of how the processes of star and planet formation occur on such a large scale. These studies include observations with the Chandra X-ray Observatory, Spitzer Space Telescope, the Herschel Space Observatory and the Gran Telescopio Canarias. As for recent observations the final stages of the process of photoablation is taking place, where the biggest stars formed and cleared the ambient material from the region.[7]

Prominent stars, including the first 12 Schulte-numbered stars
Star[8] Schulte number[8] Spectral type[9] Bolometric magnitude[9] Mass[9] (× solar)
59#1O8.5V−9.344
83[10]#2B1I−7.418
#3O6IV + O9III>17 + >8
217#4O7III((f))−9.652
V729[11][12]#5O7Ianfp + Ofpe/WN9 (+B0V:)−10.6 / −10.531 + 9
317#6O8V−9.242
457#7O3If−10.8114
465[13]#8AO6If + O5.5III(f)−9.8/−9.444 + 37
462#8BO6.5III(f)−10.480
483#8CO5If−10.171
473[14]#8DO8.5V + O9V: (+ A2V?)−8.419+19+?
431[12][15]#9O5–5.5I + O3–4III~−10/~−10>34 + >30
457#10O9.5I−10.475
734[10]#11O5If−10.058
304[16]#12B3-4Ia+−10.95110
556#18B1Ib−9.033
417#22O3If + O6V(f)~−10.4
516[10] (possible  binary[17])O5.5V((f))−11.1100
771O7V−10.690

Other noteworthy stars, not included in the Massey et al. paper include BD+40° 4210, a blue supergiant and luminous blue variable candidate, as well as the massive runaway star BD+43° 3654.

Prominent members of the association are often referred to by their Schulte numbers: for example Schulte 12, VI Cygni 12, or Cygnus OB2 #12. The numbers were first used in the 1953 discovery paper where 11 "blue giants" were numbered.[18] A 12th star (Cyg OB2 #12) was added in 1954,[19] and eight more shortly after.[20] Schulte himself maintained the already-published numbers and added many more when studying the association which he called VI Cygni.[8]

References
  1. "Ass Cyg OB 2". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2009-12-06.
  2. Rygl, K.; Brunthaler, A.; Sanna, A.; Menten, K. M.; Reid, M. J.; van Langevelde, H. J.; Honma, M.; Torstensson, K. J. E.; Fujisawa, K. (March 2012). "Parallaxes and proper motions of interstellar masers toward the Cygnus X star-forming complex. I. Membership of the Cygnus X region". Astronomy and Astrophysics. 539: A79. arXiv:1111.7023. Bibcode:2012A&A...539A..79R. doi:10.1051/0004-6361/201118211.
  3. Schuster, M. T.; Marengo, M.; Hora, J. L.; Fazio, G. G.; Humphreys, R. M.; Gehrz, R. D.; Hinz, P. M.; Kenworthy, M. A.; Hoffmann, W. F. (2009). "Imaging the Cool Hypergiant NML Cygni's Dusty Circumstellar Envelope with Adaptive Optics". The Astrophysical Journal. 699 (2): 1423–1432. arXiv:0904.4690. Bibcode:2009ApJ...699.1423S. doi:10.1088/0004-637X/699/2/1423.
  4. Knödlseder, J. (2000). "Cygnus OB2—a young globular cluster in the Milky Way". Astronomy and Astrophysics. 360: 539. arXiv:astro-ph/0007442. Bibcode:2000A&A...360..539K.
  5. Wright, Nicholas J.; Parker, Richard J.; Goodwin, Simon P.; Drake, Jeremy J. (2014). "Constraints on massive star formation: Cygnus OB2 was always an association". Monthly Notices of the Royal Astronomical Society. 438: 639–646. arXiv:1311.4537. Bibcode:2014MNRAS.438..639W. doi:10.1093/mnras/stt2232.
  6. Wright, N. J.; Drake, J. J.; Drew, J. E.; Vink, J. S. (2010). "The Massive Star-Forming Region Cygnus OB2. II. Integrated Stellar Properties and the Star Formation History". The Astrophysical Journal. 713 (2): 871–882. arXiv:1003.2463. Bibcode:2010ApJ...713..871W. doi:10.1088/0004-637X/713/2/871.
  7. Hartigan, P.; Palmer, J.; Cleeves, L.I. (2012). "Irradiated interfaces in the Ara OB1, Carina, Eagle Nebula, and Cyg OB2 massive star formation regions". High Energy Density Physics. 8 (4): 313. arXiv:1304.3930. Bibcode:2012HEDP....8..313H. doi:10.1016/j.hedp.2012.08.002.
  8. Massey, P.; Thompson, A. B. (1991). "Massive stars in CYG OB2". The Astronomical Journal. 101: 1408. Bibcode:1991AJ....101.1408M. doi:10.1086/115774.
  9. Massey, P.; Degioia-Eastwood, K.; Waterhouse, E. (2001). "The Progenitor Masses of Wolf-Rayet Stars and Luminous Blue Variables Determined from Cluster Turnoffs. II. Results from 12 Galactic Clusters and OB Associations". The Astronomical Journal. 121 (2): 1050–1070. arXiv:astro-ph/0010654. Bibcode:2001AJ....121.1050M. doi:10.1086/318769.
  10. Herrero, A.; Puls, J.; Najarro, F. (2002). "Fundamental parameters of Galactic luminous OB stars VI. Temperatures, masses and WLR of Cyg OB2 supergiants". Astronomy and Astrophysics. 396 (3): 949–966. arXiv:astro-ph/0210469. Bibcode:2002A&A...396..949H. doi:10.1051/0004-6361:20021432.
  11. Rauw, G.; Vreux, J. ‐M.; Bohannan, B. (1999). "The Interacting Early‐Type Binary BD +40°4220 (V729 Cyg): Modeling the Colliding Winds Region". The Astrophysical Journal. 517: 416–430. Bibcode:1999ApJ...517..416R. doi:10.1086/307185.
  12. Kiminki, D. C.; Kobulnicky, H. A.; Ewing, I.; Bagley Kiminki, M. M.; Lundquist, M.; Alexander, M.; Vargas-Alvarez, C.; Choi, H.; Henderson, C. B. (2012). "Additional Massive Binaries in the Cygnus OB2 Association". The Astrophysical Journal. 747: 41. arXiv:1112.3383. Bibcode:2012ApJ...747...41K. doi:10.1088/0004-637X/747/1/41.
  13. De Becker, M.; Rauw, G.; Sana, H.; Pollock, A. M. T.; Pittard, J. M.; Blomme, R.; Stevens, I. R.; Van Loo, S. (2006). "XMM-Newton observations of the massive colliding wind binary and non-thermal radio emitter CygOB2#8A [O6If + O5.5III(f)]" (PDF). Monthly Notices of the Royal Astronomical Society. 371 (3): 1280–1294. Bibcode:2006MNRAS.371.1280D. doi:10.1111/j.1365-2966.2006.10746.x.
  14. Kobulnicky, Henry A.; Kiminki, Daniel C.; Lundquist, Michael J.; Burke, Jamison; Chapman, James; Keller, Erica; Lester, Kathryn; Rolen, Emily K.; Topel, Eric; Bhattacharjee, Anirban; Smullen, Rachel A.; Vargas Alvarez, Carlos A.; Runnoe, Jessie C.; Dale, Daniel A.; Brotherton, Michael M. (2014). "Toward Complete Statistics of Massive Binary Stars: Penultimate Results from the Cygnus OB2 Radial Velocity Survey". The Astrophysical Journal Supplement Series. 213 (2): 34. arXiv:1406.6655. Bibcode:2014ApJS..213...34K. doi:10.1088/0067-0049/213/2/34.
  15. Nazé, Y.; Mahy, L.; Damerdji, Y.; Kobulnicky, H. A.; Pittard, J. M.; Parkin, E. R.; Absil, O.; Blomme, R. (2012). "The 2.35 year itch of Cygnus OB2 #9". Astronomy & Astrophysics. 546: A37. arXiv:1209.5622. Bibcode:2012A&A...546A..37N. doi:10.1051/0004-6361/201219442.
  16. Clark, J. S.; Najarro, F.; Negueruela, I.; Ritchie, B. W.; Urbaneja, M. A.; Howarth, I. D. (2012). "On the nature of the galactic early-B hypergiants". Astronomy & Astrophysics. 541: A145. arXiv:1202.3991. Bibcode:2012A&A...541A.145C. doi:10.1051/0004-6361/201117472.
  17. Kiminki, D. C.; Kobulnicky, H. A.; Kinemuchi, K.; Irwin, J. S.; Fryer, C. L.; Berrington, R. C.; Uzpen, B.; Monson, A. J.; Pierce, M. J.; Woosley, S. E. (2007). "A Radial Velocity Survey of the Cyg OB2 Association". The Astrophysical Journal. 664 (2): 1102–1120. arXiv:astro-ph/0609772. Bibcode:2007ApJ...664.1102K. doi:10.1086/513709.
  18. Münch, Luis; Morgan, W. W. (1953). "Notes: A Probable Clustering of Blue Giants in Cygnus". Astrophysical Journal. 118: 161. Bibcode:1953ApJ...118..161M. doi:10.1086/145737.
  19. Morgan, W. W.; Johnson, H. L.; Roman, Nancy G. (1954). "A Very Red Star of Early Type in Cygnus". Publications of the Astronomical Society of the Pacific. 66: 85. Bibcode:1954PASP...66...85M. doi:10.1086/126660.
  20. Morgan, W. W.; Meinel, A. B.; Johnson, Hugh M. (1954). "Spectral Classification with Exceedingly Low Dispersion". Astrophysical Journal. 120: 506. Bibcode:1954ApJ...120..506M. doi:10.1086/145938.
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