WR 21a

WR 21a is an eclipsing 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

Location of WR 21a (circled) near RCW 49 Credit: ESO
Observation data Epoch J2000      Equinox J2000
Constellation	Carina[1]
Right ascension	10h 25m 56.502s[2]
Declination	−57° 48′ 43.52″[2]
Apparent magnitude (V)	12.661[3]
Characteristics
Spectral type	O3/WN5ha + O3Vz((f*))[4]
B−V color index	+1.507[3]
Variable type	Eclipsing[5]
Astrometry
Proper motion (μ)	RA: −4.079[2] mas/yr Dec.: 2.617[2] mas/yr
Parallax (π)	0.2497±0.0143 mas[2]
Distance	13,100 ± 700 ly (4,000 ± 200 pc)
Absolute magnitude (MV)	−7.20[4]
Orbit[5]
Primary	WR
Companion	O
Period (P)	31.67855±0.00002 days
Semi-major axis (a)	230 R☉[6]
Eccentricity (e)	0.695±0.007
Inclination (i)	61.8±1.5°
Semi-amplitude (K1) (primary)	158.0±2.7 km/s
Semi-amplitude (K2) (secondary)	278.1±2.8 km/s
Details
WR
Mass	94.4[5] M☉
Radius	23.3±1.6[5] R☉
Luminosity	1,510,000[5] L☉
Surface gravity (log g)	3.69[5] cgs
Temperature	45,000[7] K
O
Mass	53.6[5] M☉
Radius	14.8±2.0[5] R☉
Luminosity	1,050,000[5] L☉
Surface gravity (log g)	3.81[5] cgs
Temperature	50,680[7] K
Age	1.5[4] Myr
Other designations
WR 21a, 2MASS J10255650-5748435
Database references
SIMBAD	data

WR 21a lies near the Westerlund 2 open cluster^[8] and likely to be an ejected member.^[9]

The distance of WR 21a was not definitively known until Gaia mission. There have been estimates from 2.85 kpc to around 8 kpc, with consequent uncertainties in the system luminosity. The larger distance was preferred because of consistency with the derived orbital parameters.^[7][10]

Every 31 days and 16 hours the two stars in this system revolve around each other. The inclination of the orbit means that only very shallow eclipses are observed and the brightness dips by only about 0.05 magnitudes. There are also even smaller brightness variations attributed to the heartbeat effect where the closest passage of the stars in their eccentric orbits creates brightness changes as the two stars illuminate each other. There may also be tidally-excited oscillations producing further small variations.^[5]

The colliding winds of the two stars produce extremely high temperatures and luminous x-ray emission.^[6] The system is also bright at radio wavelengths.^[7]