Westerlund 1-237 or Wd 1-237 is a possible red supergiant (RSG) in the constellation of Ara. It is one out of 4 known red supergiants in the Westerlund 1 super star cluster, although its outlying position, spectrum, and parallax, suggest it could be a foreground giant.[10] As a red supergiant, it would be one of the largest known stars and one of most luminous of its type.[7]

Westerlund 1-237

Westerlund 1 super star cluster. The location of Wd 1-237 is circled. Credit: ESO
Observation data Epoch J2000      Equinox J2000
Constellation	Ara
Right ascension	16h 47m 03.1075s
Declination	−45° 52′ 18.9572″
Apparent magnitude (V)	19.008[1]
Characteristics
Evolutionary stage	either a red supergiant or a foreground giant[2]
Spectral type	M3Ia[3] or M6+ III[4]
Apparent magnitude (B)	22.8[citation needed]
Apparent magnitude (J)	5.075[5]
Apparent magnitude (H)	3.01[5]
Apparent magnitude (K)	2.18[5]
Apparent magnitude (G)	11.259[6]
Apparent magnitude (R)	13.634[1]
Astrometry
Proper motion (μ)	RA: -3.09±0.15[6] mas/yr Dec.: -4.12±0.12[6] mas/yr
Parallax (π)	0.3370 ± 0.1235 mas[6]
Distance	3,000[7] or 623[8] pc
Absolute magnitude (MV)	–6.96[9]
Details
if a supergiant
Radius	1,241±70[9] R☉
Luminosity	219,000±26,000[9] L☉
Temperature	3,550[9] - 3,605[8] K
Age	7.9[9] Myr
if a foreground giant
Radius	216[8] R☉
Luminosity	1,000[2] or 7,178 – 7,379[8] L☉
Temperature	3,605[8] K
Other designations
Westerlund 1 W237, Westerlund 1 BKS B, 2MASS J16470309-4552189, Gaia EDR3 5940105904023386752
Database references
SIMBAD	data

Physical characteristics

Westerlund 1-237 is classified as a luminous cool supergiant emitting most of its energy in the infrared spectrum.[11] It is surrounded by a radio nebula which is similar in mass to those of Westerlund 1-20 and Westerlund 1-26, and moreover directly comparable to that of VY Canis Majoris. The elliptical structure of this nebula however indicates that it has been less affected by the cluster wind of Westerlund 1 (W20 and W26 have pronounced cometary shaped nebulae). The outflow velocity for the RSG wind is assumed to be around 30 km/s. The nebula itself seems to have a mass of 0.07 M_☉ and a radius of about 0.11 parsecs. This results in a kinematic age around 3,600 years and a time averaged mass loss rate of 2×10⁻⁵ M_☉ per year.[12]

The star occupies the upper right corner of the Hertzsprung-Russell diagram. With an effective temperature of 3,550 K and a bolometric luminosity of 219,000 L_☉, the radius of Westerlund 1-237 would be 1,241 times the solar radius (R_☉), making it larger than the orbit of Jupiter. The initial mass of W237 has been calculated from its position relative to theoretical stellar evolutionary tracks to be around 28 M_☉ or 32 M_☉ for a non-rotating star.[9]

Distance

The distance of Westerlund 1-237 is assumed to be around 8,500+2,000
−1,300 light years or 2,600+600
−400 parsecs[13] based on it being commonly thought of as a member of the Westerlund 1 star cluster (the elliptical shape of its nebula indicates that it might not be near the center of W1, while other RSGs like W20 and W26 are).[12] Another but older source suggests a similar distance of 3,000±500 parsecs.[7]

Westerlund's 1987 analysis assigned a spectral type of M6⁺ III to W1-237 and considered it to be a foreground giant with a luminosity only around 1,000 L_☉.[2] Gaia Data Release 2 gives a parallax of 1.64±0.2608 mas for W1-237, implying a distance of 623+139
−96 pc and a luminosity of 7,178–7,379 L_☉ with a corresponding radius of 216 R_☉.[14][8] In 2020, the parallax was revised to the much smaller value of 0.3370±0.1235 mas.[6]

See also

• Westerlund 1-75

References

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