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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
Declination	−57° 45′ 48.99″
Apparent magnitude (V)	13.28
Characteristics
Evolutionary stage	Hot slash star
Spectral type	O3If/WN6 + O3If/WN6
U−B color index	+0.29
B−V color index	+1.63
Variable type	β Lyr + WR
Astrometry
Proper motion (μ)	RA: −5.391 mas/yr ; Dec.: 2.949 mas/yr
Parallax (π)	0.2310 ± 0.0171 mas
Distance	14,000 ± 1,000 ly ; (4,300 ± 300 pc)
Absolute magnitude (MV)	−6.49 / −6.49
Orbit
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
Primary
Mass	82.7±5.5 M☉
Radius	19.3±0.5 R☉
Luminosity	1,150,000±150,000 L☉
Surface gravity (log g)	3.8 cgs
Temperature	43,000±2,000 K
Secondary
Mass	81.9±5.5 M☉
Radius	19.3±0.5 R☉
Luminosity	1,150,000±150,000 L☉
Surface gravity (log g)	3.8 cgs
Temperature	43,000±2,000 K
Other designations
	V712 Carinae, THA 35-II-36, Westerlund 2 MSP 240, 2MASS J10235800-5745489, UBV M 40466, Westerlund 2 4
Database references
SIMBAD	data

WR 20a is an eclipsing 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]

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.

A visual band light curve for WR 20a (V712 Carinae), adapted from Kochanek *et al.* (2017)^[10]

Every 3.6 days the two stars in this system revolve around each other. Although the stars are in a very tight orbit, both stars in the system are detached.^[11] The two stars eclipse each other on each orbit, producing a drop in brightness of about 0.4 magnitudes. The brightness is also continuously variable outside the eclipses due to the distorted shapes of the two stars.^[9] The primary and secondary minima are almost the same depth since the size and temperature of each star is almost identical.^[2]

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]^[12] The X-ray emitting region is quite extended since it does not suffer from any eclipse.

References

^ ^a ^b Cutri, Roc M.; Skrutskie, Michael F.; Van Dyk, Schuyler D.; Beichman, Charles A.; Carpenter, John M.; Chester, Thomas; Cambresy, Laurent; Evans, Tracey E.; Fowler, John W.; Gizis, John E.; Howard, Elizabeth V.; Huchra, John P.; Jarrett, Thomas H.; Kopan, Eugene L.; Kirkpatrick, J. Davy; Light, Robert M.; Marsh, Kenneth A.; McCallon, Howard L.; Schneider, Stephen E.; Stiening, Rae; Sykes, Matthew J.; Weinberg, Martin D.; Wheaton, William A.; Wheelock, Sherry L.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues. 2246: II/246. Bibcode:2003yCat.2246....0C.
^ ^a ^b ^c ^d ^e ^f ^g ^h 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.
^ 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.
^ 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.
^ 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.
^ ^a ^b ^c Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
^ ^a ^b ^c 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.
^ Rate, Gemma; Crowther, Paul A.; Parker, Richard J. (2020), "Unlocking Galactic Wolf–Rayet stars with Gaia DR2 – II. Cluster and association membership", Monthly Notices of the Royal Astronomical Society, 495: 1209–1226, arXiv:2005.02533, Bibcode:2020MNRAS.495.1209R, doi:10.1093/mnras/staa1290
^ ^a ^b 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.
^ Kochanek, C. S.; Shappee, B. J.; Stanek, K. Z.; Holoien, T. W.-S.; Thompson, Todd A.; Prieto, J. L.; Dong, Subo; Shields, J. V.; Will, D.; Britt, C.; Perzanowski, D.; Pojmański, G. (October 2017). "The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0". Publications of the Astronomical Society of the Pacific. 129 (980): 104502. arXiv:1706.07060. Bibcode:2017PASP..129j4502K. doi:10.1088/1538-3873/aa80d9.
^ 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.
^ 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.