Westerlund 1 W237 compared to the other 3 RSGs (Red Supergiants) in theWesterlund 1 star cluster.
Westerlund 1 W237 is classified as a luminous coolsupergiant emitting most of its energy in theinfrared spectrum.[11] It is surrounded by a radio nebula which is similar in mass to those ofWesterlund 1-20 andWesterlund 1 W26, and moreover directly comparable to that ofVY Canis Majoris. The elliptical structure of this nebula however indicates that it has been less affected by the cluster wind ofWesterlund 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 of2×10−5M☉ per year.[12]
The star occupies the upper right corner of theHertzsprung-Russell diagram. With an effective temperature of 3,550 K and abolometric luminosity of 219,000 L☉, the radius of Westerlund 1 W237 would be 1,241 times thesolar radius (R☉), making it larger than the orbit ofJupiter. The initial mass of W237 has been calculated from its position relative to theoreticalstellar evolutionary tracks to be around 28 M☉ or 32 M☉ for a non-rotating star.[10]
The distance of Westerlund 1 W237 is assumed to be around8,500+2,000 −1,300light years or2,600+600 −400parsecs[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 of3,000±500 parsecs.[8]
Westerlund's 1987 analysis assigned a spectral type of M6+ III to W1-237 and considered it to be a foregroundgiant with a luminosity only around 1,000 L☉.[4]Gaia Data Release 2 gives a parallax of1.64±0.2608 mas for W237, implying a distance of623+139 −96pc and a luminosity of 7,178–7,379 L☉ with a corresponding radius of 216 R☉.[14][9] In 2020, the parallax was revised to the much smaller value of 0.3370±0.1235 mas, corresponding to a distance of2967±1087 pc (9670±3540 ly).[2]
^abcWesterlund, B. E. (1987). "Photometry and spectroscopy of stars in the region of a highly reddened cluster in ARA".Astronomy and Astrophysics. Supplement.70 (3):311–324.Bibcode:1987A&AS...70..311W.ISSN0365-0138.
^abcCutri, 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.
^abcFok, Thomas K. T.; Nakashima, Jun-Ichi; Yung, Bosco H. K.; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters".The Astrophysical Journal.760 (1): 65.arXiv:1209.6427.Bibcode:2012ApJ...760...65F.doi:10.1088/0004-637X/760/1/65.S2CID53393926.
