WR 120

WR 120 is a binary containing two Wolf-Rayet stars in the constellation of Scutum, around 10,000 light years away. The primary is a hydrogen-free weak-lined WN7 star, the secondary is a hydrogen-free WN3 or 4 star, and the system is a possible member of the cluster Dolidze 33.^[8] From our point of view, WR 120 is reddened by 4.82 magnitudes.

WR 120

A near-infrared (2.033 micron) light curve for V462 Scuti, adapted from Chené and St-Louis (2007).[1] The left-most point shows the 3 sigma error bar.
Observation data Epoch J2000      Equinox J2000
Constellation	Scutum
Right ascension	18h 41m 00.86698s[2]
Declination	−4° 26′ 14.47345″[2]
Apparent magnitude (V)	11.93[3]
Characteristics
Evolutionary stage	Wolf-Rayet
Spectral type	WN7w[4] + WN3/4[5]
U−B color index	0.13[6]
B−V color index	1.04[6]
Astrometry
Proper motion (μ)	RA: −0.489[7] mas/yr Dec.: −2.524[7] mas/yr
Parallax (π)	0.3246±0.0541 mas[7]
Distance	approx. 10,000 ly (approx. 3,100 pc)
Absolute magnitude (MV)	−3.81[4]
Details
Mass	7[4] M☉
Radius	3.78[4] R☉
Luminosity	83,200[4] L☉
Temperature	50,100[4] K
Other designations
MR 89, V462 Scuti, 2MASS J18410086-0426145
Database references
SIMBAD	data

Photometric observations obtained in 1995 by Sergey V. Marchenko et al. showed that WR 120 is a variable star.^[9] For that reason it was given its variable star designation, V462 Scuti, in the year 2000.^[10]

Properties

Analysis of the primary's spectrum with PoWR shows that it has a temperature of around 50,000 Kelvins, and is losing mass at a rate of 10^−4.9 M_☉/year, or 1 solar mass every 80,000 years, which is being carried away from the surface at a speed of 1,225 kilometres per second.^[4] Taking its close distance into account, WR 120 A's luminosity turns out to be a mere 83,200 L_☉, which would make it one of the dimmest WN stars known, and one of the only WN stars with a luminosity below 100,000 L_☉. Using the Stefan-Boltzmann Law, a radius of 3.78 R_☉ is derived, and a "transformed" radius at an optical depth of 2/3, more comparable to other types of stars, is at about 6 R_☉. Using the WR Luminosity-Mass ratio, WR 120 may have a mass of just 7 M_☉, one of the lowest masses of any WR star. WR 120 A’s visual luminosity is 2,858 L_☉, which is also on the lower end of WR visual luminosities.^[4]

WR 120 is thought to be a member of Dolidze 33, an open cluster nearly 3,000 pc away.^[8]

Binarity

In 2021, WR 120 was revealed to be a binary star. Previously, it was thought to be a single WR star, but it is in fact a rare double Wolf-Rayet star. The companion (a WN3/4 WR star) is located approximately 1700 AU from the primary WN7 WR, and is about 2.1 magnitudes fainter than WR 120.