Plaskett's Star

Plaskett's Star /ˈplæskɪts/, also known as HR 2422 and V640 Monocerotis, is a spectroscopic binary at a distance of around 6600 light-years. It is one of the most massive binary stars known, with a total mass of around one hundred times that of the Sun.^[7] Indeed, it was long thought to be the most massive known binary system,^[8][9] but evidence collected between 1996 and 2005 demonstrated that Eta Carinae, which was previously thought to be a massive individual star, is a binary system.^[10]

Plaskett's star

Location of Plaskett's star (circled)
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Monoceros
Right ascension	06h 37m 24.04209s[1]
Declination	+06° 08′ 07.3691″[1]
Apparent magnitude (V)	6.04 - 6.08[2]
Characteristics
Spectral type	O8I + O7.5III[3]
U−B color index	–0.88[4]
B−V color index	+0.05[4]
Astrometry
Radial velocity (Rv)	+24.5[5] km/s
Proper motion (μ)	RA: –2.070[1] mas/yr Dec.: +1.226[1] mas/yr
Parallax (π)	0.7615±0,0701 mas[1]
Distance	5,245 ly (1,608[6] pc)
Details
A
Mass	54[7] M☉
Radius	14.2[7] R☉
Luminosity	224,000[7] L☉
Surface gravity (log g)	3.5±0.1[7] cgs
Temperature	33,500±2,000[7] K
Rotational velocity (v sin i)	75[8] km/s
B
Mass	56[7] M☉
Radius	10.8[7] R☉
Luminosity	123,000[7] L☉
Surface gravity (log g)	3.5±0.1[7] cgs
Temperature	33,000±2,000[7] K
Rotational velocity (v sin i)	300[8] km/s
Other designations
V640 Monocerotis, HR 2422, BD+6°1309, GC 8631, HIP 31646 , HD 47129.
Database references
SIMBAD	data

This system is named after John Stanley Plaskett, the Canadian astronomer who discovered its binary nature in 1922. Plaskett was assisted in his observations by his son, Harry Hemley Plaskett. The pair of stars have a combined visual magnitude of 6.05, and are located in the constellation of Monoceros.

A light curve for V640 Monocerotis, plotted from TESS data^[11]

The orbital period for the pair is 14.39625±0.00095 d.^[7] The secondary is a rapid rotator with a projected rotational velocity of 300 km·s⁻¹,^[8] giving it a pronounced equatorial bulge.^[7] There is a companion star at a projected angular separation of 32.29 mas, as well as two visual companions separated by 0.78″ and 1.12″, respectively.^[12]

The brightness varies irregularly from 6.0 to 6.1 on a timescale of a few hours, thought to be due to many factors including the binary orbit, hot spots in the colliding winds, and granulation.^[3]

The luminosities of each component, assuming membership of the Monoceros OB2 association, are much lower than expected for their spectral types. It has been suggested that the star may be further away, and not a member of the Monoceros OB2 association. The masses derived from the binary orbit are also somewhat higher than expected from the spectral types, but with considerable uncertainty due to assumptions about the inclination.^[3] The Gaia Data Release 3 parallax supports the closer distance and membership of Monoceros OB2.^[1]