Cygnus OB2-12

Cygnus OB2 #12 is an extremely bright blue hypergiant with an absolute bolometric magnitude (all electromagnetic radiation) of −10.9, among the most luminous stars known in the galaxy. This makes the star nearly two million times more luminous than the Sun, although less than half the estimates when the star was first discovered. It is now known to be a binary, with the companion approximately a tenth as bright. A very approximate initial estimate of the orbit gives the total system mass as 120 M☉ and the period as 30 years.

Cygnus OB2 #12
Location of Cygnus OB2 #12 (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Cygnus
Right ascension 20h 32m 40.9589s[1]
Declination 41° 14′ 29.286″[1]
Characteristics
Spectral type B3-4 Ia+[2]
Apparent magnitude (B) 14.41[3]
Apparent magnitude (V) 11.40[3]
Apparent magnitude (J) 4.667±0.324[4]
Apparent magnitude (H) 3.512±0.260[4]
Apparent magnitude (K) 2.704±0.364[4]
U−B color index 1.69[3]
B−V color index 3.01[3]
Variable type cLBV
Astrometry
Radial velocity (Rv)−11.00[5] km/s
Proper motion (μ) RA: −1.871[6] mas/yr
Dec.: −3.210[6] mas/yr
Parallax (π)1.1751 ± 0.1285[6] mas
Distance1,750[7] pc
Absolute magnitude (MV)−9.82[7]
Details
Mass110[8] M☉
Radius246[8] R☉
Luminosity1,660,000[7] L☉
Temperature13,700[8] K
Age3.0[8] Myr
Other designations
Cyg OB2 #12, Schulte 12, 2MASS J20324096+4114291, NSV 13138, HIP 101364, Gaia DR2 2067782734461462912
Database references
SIMBADdata

Cygnus OB2

Cygnus OB2 #12 is generally assumed to be a member of the Cyg OB2 Association, a cluster of young massive stars about 5,740 light years away in Cygnus, and resides in a region of the Milky Way from which visible light is heavily absorbed by interstellar dust when viewed from Earth. The dust causes the star to be strongly reddened despite being an intrinsically hot and blue star, hence it has been extensively studied in the infra-red. Were it not for the dust extinction, the star would have a visual magnitude about 1.5, nearly as bright as Deneb (Alpha Cygni), but because of the dust the observed visual magnitude is 11.4 so that it requires binoculars or a small telescope to be seen.[2]

There are several fainter stars around Cygnus OB2 #12, thought to comprise a small cluster. Two stars are only resolved by speckle interferometry. One is thought to be a main sequence B star in a 100-200 year orbit. It is 2.3 magnitudes fainter than the primary star and 0.063" away. The other is 4.8 magnitudes fainter and about an arc-second distant. No spectroscopic companion could be detected in the spectrum.[9] However, analysis of x-ray emission suggests that there may be a hot luminous companion in a close orbit, with their colliding winds creating the x-rays.[7]

Distance

The Gaia Data Release 2 parallax for Cygnus OB2 #12 indicates a distance around 840 pc and is inconsistent with the distance of other Cygnus OB2 members.[10] However, the result includes a significant amount of noise suggesting that it may be inaccurate.[11] Earlier measures of the parallax were too imprecise to be meaningful.[1] If the closer distance is accepted then the star would be a more conventional blue supergiant with a luminosity around 440,000 L☉, a radius of 118 R☉, and a current mass of 25 M☉.[10]

Properties

Cygnus OB2 #12 is a candidate luminous blue variable (LBV). Its position in the HR diagram, luminosity, and spectrum all classify it as an LBV. It shows brightness variations of a few tenths of a magnitude, but these do not seem to be associated with colour changes that would be expected from an LBV.[12] The spectral type has varied slightly since its discovery, but not to the extent that would be normal for an LBV.[8] If the lower Gaia DR2 distance were correct then the properties of Cygnus OB2 #12 would mean it would be expected to pulsate at low amplitude with a period of a few days.[10]

References
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. Caballero-Nieves, S. M.; Nelan, E. P.; Gies, D. R.; Wallace, D. J.; Degioia-Eastwood, K.; Herrero, A.; Jao, W.-C.; Mason, B. D.; Massey, P.; Moffat, A. F. J.; Walborn, N. R. (2014). "A High Angular Resolution Survey of Massive Stars in Cygnus OB2: Results from the Hubble Space Telescope Fine Guidance Sensors". The Astronomical Journal. 147 (2): 40. arXiv:1311.5087. Bibcode:2014AJ....147...40C. doi:10.1088/0004-6256/147/2/40. S2CID 22036552.
  3. Reed, B. Cameron (2003). "Catalog of Galactic OB Stars". The Astronomical Journal. 125 (5): 2531–2533. Bibcode:2003AJ....125.2531R. doi:10.1086/374771.
  4. Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R. M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.; Wheelock, S.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally Published in: 2003yCat.2246....0C. 2246: 0. Bibcode:2003yCat.2246....0C.
  5. Klochkova, V. G.; Chentsov, E. L. (2004). "The Optical Spectrum of an LBV Candidate in the Cyg OB2 Association". Astronomy Reports. 48 (12): 1005–1018. arXiv:astro-ph/0605483. Bibcode:2004ARep...48.1005K. doi:10.1134/1.1836024. S2CID 55121658.
  6. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  7. Oskinova, L. M.; Huenemoerder, D. P.; Hamann, W. -R.; Shenar, T.; Sander, A. A. C.; Ignace, R.; Todt, H.; Hainich, R. (2017). "On the Binary Nature of Massive Blue Hypergiants: High-resolution X-Ray Spectroscopy Suggests That Cyg OB2 12 is a Colliding Wind Binary". The Astrophysical Journal. 845 (1): 39. arXiv:1707.04473. Bibcode:2017ApJ...845...39O. doi:10.3847/1538-4357/aa7e79. S2CID 119537489.
  8. 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. S2CID 11978733.
  9. Maryeva, O. V.; Chentsov, E. L.; Goranskij, V. P.; Dyachenko, V. V.; Karpov, S. V.; Malogolovets, E. V.; Rastegaev, D. A. (2016). "On the nature of high reddening of Cygnus OB2 #12 hypergiant". Monthly Notices of the Royal Astronomical Society. 1602 (1): 491–507. arXiv:1602.05042. Bibcode:2016MNRAS.458..491M. doi:10.1093/mnras/stw385. S2CID 118824868.
  10. Nazé, Yaël; Rauw, Gregor; Czesla, Stefan; Mahy, Laurent; Campos, Fran (2019). "Variations on a theme: The puzzling behaviour of Schulte 12". Astronomy and Astrophysics. 627: A99. arXiv:1906.00762. Bibcode:2019A&A...627A..99N. doi:10.1051/0004-6361/201935141. S2CID 173990444.
  11. Berlanas, S. R.; Wright, N. J.; Herrero, A.; Drew, J. E.; Lennon, D. J. (2019). "Disentangling the spatial substructure of Cygnus OB2 from Gaia DR2". Monthly Notices of the Royal Astronomical Society. 484 (2): 1838. arXiv:1901.02959. Bibcode:2019MNRAS.484.1838B. doi:10.1093/mnras/stz117. S2CID 118869784.
  12. Chentsov, E. L.; Klochkova, V. G.; Panchuk, V. E.; Yushkin, M. V.; Nasonov, D. S. (2013). "Spectroscopy of 13 high-mass stars in the Cyg OB2 association". Astronomy Reports. 57 (7): 527–547. arXiv:1306.1087. Bibcode:2013ARep...57..527C. doi:10.1134/S1063772913070019. S2CID 123003317.
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