WR 21a

WR 21a is a binary star in the constellation Carina. It includes one of the most massive known stars and is one of the most massive binaries.

WR 21a lies near the Westerlund 2 open cluster and may be an ejected member.[6]

The distance of WR 21a is not definitely known. There have been estimates from 2.85 kpc to around 8 kpc, with consequent uncertainties in the system luminosity. The larger distance is preferred because of consistency with the derived orbital parameters,[5] but it is still uncertain whether the star is nearer or further than Westerlund 2.[7]

Every 31 days and 16 hours days the two stars in this system revolve around each other. The inclination of the orbit means that we do not see eclipses and the visual brightness of the system is quite stable.[3]

The colliding winds of the two stars produce extremely high temperatures and luminous x-ray emission.[4] The system is also bright at radio wavelengths.[5]

WR 21a
Westerlund 2
Green circle.png
Location of WR 21a (circled) near RCW 49
Credit: ESO
Observation data
Epoch J2000      Equinox J2000
Constellation Carina
Right ascension 10h 25m 56.50s[1]
Declination −57° 48′ 43.5″[1]
Apparent magnitude (V) 12.661[2]
Spectral type O3/WN5ha + O3Vz((f*))[3]
B−V color index +1.507[2]
Proper motion (μ) RA: −2.5[2] mas/yr
Dec.: +2.1[2] mas/yr
Distance ~8,000[3] pc
Absolute magnitude (MV) −7.20[3]
Primary WR
Companion O
Period (P) 31.672 ± 0.011 days
Semi-major axis (a) 230 R[4]
Eccentricity (e) 0.6949 ± 0.0047
Inclination (i) 58.8 ± 2.5°
Semi-amplitude (K1)
156.9 ± 2.0 km/s
Semi-amplitude (K2)
278.6 ± 5.3 km/s
Mass 103.6 ± 10.2[3] M
Radius 12.0[4] R
Luminosity 1,800,000[3] L
Temperature 45,000[5] K
Mass 58.3 ± 3.7[3] M
Radius 13.84[4] R
Luminosity 630,000[3] L
Temperature 50,680[5] K
Age 1.5[3] Myr
Other designations
WR 21a, 2MASS J10255650-5748435
Database references


  1. ^ a b 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. Bibcode:2003yCat.2246....0C.
  2. ^ a b c d Zacharias, N.; Finch, C. T.; Girard, T. M.; Henden, A.; Bartlett, J. L.; Monet, D. G.; Zacharias, M. I. (2013). "The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)". The Astronomical Journal. 145 (2): 44. arXiv:1212.6182Freely accessible. Bibcode:2013AJ....145...44Z. doi:10.1088/0004-6256/145/2/44.
  3. ^ a b c d e f g h i j Tramper, F.; Sana, H.; Fitzsimons, N. E.; De Koter, A.; Kaper, L.; Mahy, L.; Moffat, A. (2016). "The mass of the very massive binary WR21a". Monthly Notices of the Royal Astronomical Society. 455 (2): 1275–1281. arXiv:1510.03609Freely accessible. Bibcode:2016MNRAS.455.1275T. doi:10.1093/mnras/stv2373.
  4. ^ a b c d Gosset, Eric; Naze, Yael (2016). "The X-ray light curve of the massive colliding wind Wolf-Rayet + O binary WR21a". Astronomy & Astrophysics. 1604: A113. arXiv:1604.01536Freely accessible. Bibcode:2016A&A...590A.113G. doi:10.1051/0004-6361/201527051.
  5. ^ a b c d Benaglia, P.; Romero, G. E.; Koribalski, B.; Pollock, A. M. T. (2005). "Multiwavelength studies of WR 21a and its surroundings". Astronomy and Astrophysics. 440 (2): 743–750. arXiv:astro-ph/0506363Freely accessible. Bibcode:2005A&A...440..743B. doi:10.1051/0004-6361:20042617.
  6. ^ Hur, Hyeonoh; Park, Byeong-Gon; Sung, Hwankyung; Bessell, Michael S.; Lim, Beomdu; Chun, Moo-Young; Sohn, Sangmo Tony (2015). "Reddening, distance, and stellar content of the young open cluster Westerlund 2". Monthly Notices of the Royal Astronomical Society. 446 (4): 3797–3819. arXiv:1411.0879Freely accessible. Bibcode:2015MNRAS.446.3797H. doi:10.1093/mnras/stu2329.
  7. ^ Carraro, G.; Turner, D.; Majaess, D.; Baume, G. (2013). "The distance to the young open cluster Westerlund 2". Astronomy & Astrophysics. 555: A50. arXiv:1305.4309Freely accessible. Bibcode:2013A&A...555A..50C. doi:10.1051/0004-6361/201321421.

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