WR 20a

WR20a is a binary star belonging to or recently (0.5 millions years before present) ejected from the young, massive cluster Westerlund 2.[8] It was discovered in 2004 to be one of the most massive binary systems known, for which the masses of the components have been accurately measured.[7]

WR 20a
WR 20a (circled) near Westerlund 2
Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team
Observation data
Epoch J2000      Equinox J2000
Constellation Carina
Right ascension 10h 23m 58.002s[1]
Declination −57° 45′ 48.99″[1]
Apparent magnitude (V) 13.28[2]
Characteristics
Evolutionary stage Hot slash star[3]
Spectral type O3If*/WN6 + O3If*/WN6[4]
U−B color index +0.29[2]
B−V color index +1.63[2]
Variable type β Lyr + WR[5]
Astrometry
Proper motion (μ) RA: -5.391[6] mas/yr
Dec.: 2.949[6] mas/yr
Parallax (π)0.2310 ± 0.0171[6] mas
Distance14,000 ± 1,000 ly
(4,300 ± 300 pc)
Absolute magnitude (MV)−6.49 + −6.49[2]
Orbit[7]
Period (P)3.675 days
Semi-major axis (a)26 R☉
Eccentricity (e)0
Semi-amplitude (K1)
(primary)		353.1 km/s
Semi-amplitude (K2)
(secondary)		362.6 km/s
Details[2]
Mass82.7 ± 5.5 M☉
Radius19.3 ± 0.5 R☉
Luminosity1,150,000 ± 150,000 L☉
Surface gravity (log g)3.8 cgs
Temperature43000 ± 2000 K
Mass81.9 ± 5.5 M☉
Radius19.3 ± 0.5 R☉
Luminosity1,150,000 ± 150,000 L☉
Surface gravity (log g)3.8 cgs
Temperature43000 ± 2000 K
Other designations
V712 Carinae, GEN#+4.38020004, THA 35-II-36, Westerlund 2 MSP 240, 2MASS J10235800-5745489, UBV M 40466, Westerlund 2 4
Database references
SIMBADdata

Each star in the system has about eighty times the mass of the Sun.[7][9] It is not clear why this system is located away from the center of the cluster. It is possible that the system was formed in the core, but that it was ejected by dynamical interactions.

Every 3.6 days the two stars in this system revolve around each other. Although the stars are in very tight orbit, both stars in the system are detached.[10]

It is expected that within a million years the two will expand and come into contact. A large nitrogen abundance has been measured on the surface of the stars,[2] about six times the abundance of nitrogen measured in the sun. This nitrogen is probably produced in deeper layers of the star and pushed towards the surface by rotational mixing.

A collision between the two winds of the systems has been detected in the visible as well as in X-rays.[2][11] The X-ray emitting region is quite extended since it does not suffer from any eclipse.

References
  1. 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. Rauw; et al. (4 March 2005). "The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions" (PDF). Astronomy & Astrophysics. 432 (3): 985–998. Bibcode:2005A&A...432..985R. doi:10.1051/0004-6361:20042136.
  3. Crowther, Paul A.; Walborn, Nolan R. (2011). "Spectral classification of O2-3.5 If*/WN5-7 stars". Monthly Notices of the Royal Astronomical Society. 416 (2): 1311. arXiv:1105.4757. Bibcode:2011MNRAS.416.1311C. doi:10.1111/j.1365-2966.2011.19129.x. S2CID 118455138.
  4. Roman-Lopes, A. (2013). "An O2 If*/WN6 star caught in the act in a compact H II region in the starburst cluster NGC 3603". Monthly Notices of the Royal Astronomical Society. 433 (1): 712–718. arXiv:1305.0851. Bibcode:2013MNRAS.433..712R. doi:10.1093/mnras/stt762. S2CID 118367798.
  5. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  6. Brown, A. G. A.; et al. (Gaia collaboration). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics (in press). arXiv:2012.01533. doi:10.1051/0004-6361/202039657. S2CID 227254300. Gaia EDR3 record for this source at VizieR.
  7. Rauw; et al. (3 June 2004). "WR20a: a massive cornerstone binary system comprising two extreme early-type stars". Astronomy & Astrophysics. 420 (2): L9–L13. arXiv:astro-ph/0404551. Bibcode:2004A&A...420L...9R. doi:10.1051/0004-6361:20040150. S2CID 119426384.
  8. Unlocking Galactic Wolf-Rayet stars with Gaia DR2 II: Cluster and Association membership, 2020, arXiv:2005.02533
  9. Bonanos, A. Z.; Stanek, K. Z.; Udalski, A.; Wyrzykowski, L.; Żebruń, K.; Kubiak, M.; Szymański, M. K.; Szewczyk, O.; Pietrzyński, G.; Soszyński, I. (2004). "WR 20a is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System". The Astrophysical Journal. 611 (1): L33–L36. arXiv:astro-ph/0405338. Bibcode:2004ApJ...611L..33B. doi:10.1086/423671. S2CID 119479873.
  10. Rauw; et al. (1 March 2007). "Early-type stars in the core of the young open cluster Westerlund 2". Astronomy & Astrophysics. 463 (3): 981–991. arXiv:astro-ph/0612622. Bibcode:2007A&A...463..981R. doi:10.1051/0004-6361:20066495. S2CID 17776145.
  11. Naze; et al. (1 May 2008). "Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund 2". Astronomy & Astrophysics. 483 (1): 171–182. arXiv:0801.0647. Bibcode:2008A&A...483..171N. doi:10.1051/0004-6361:20078851. S2CID 17739235.
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