Westerlund 1-20

Westerlund 1-20 or Wd 1-20 is a red supergiant (RSG) located in the Westerlund 1 super star cluster. Its radius was calculated to be around 965 solar radii[1] (6.72 × 108 km, 4.48 au), making it one of the largest stars discovered so far. This corresponds to a volume 899 million times bigger than the Sun. If placed at the center of the Solar System, the photosphere of Westerlund 1-20 would almost reach the orbit of Jupiter.

Westerlund 1-20 compared to the other 3 RSGs in the Westerlund 1 star cluster.

Westerlund 1-20
Westerlund 1 super star cluster. The location of Wd 1-20 is circled.
Credit: ESO
Observation data
Epoch J2000      Equinox J2000
Constellation Ara
Right ascension 16h 47m 04.69s[1]
Declination −45° 51′ 23.9″[1]
Characteristics
Evolutionary stage Red supergiant
Spectral type M3[2] - M6I[3]
Apparent magnitude (J) 6.378
Apparent magnitude (H) 4.381
Apparent magnitude (K) 2.61
Astrometry
Proper motion (μ) RA: -2.478[4] mas/yr
Dec.: -4.041[4] mas/yr
Parallax (π)0.1690 ± 0.1073[4] mas
Distance2,600+600
−400 (assumed)[5] pc
Details
Radius858±48[2] R☉
Luminosity101,000±13,000[2] L☉
Temperature3,284 - 3,550[2] K
Other designations
Westerlund 1 W20, Westerlund 1 BKS D, 2MASS J16470468-4551238, Gaia EDR3 5940106007092341120
Database references
SIMBADdata

The star is classified as a luminous cool supergiant emitting most of its energy in the infrared spectrum.[6] W20 occupies the upper right corner of the Hertzsprung-Russell diagram. Using the effective temperature of 3,500 K,[1] the bolometric luminosity of 126,000 L☉[1] and the solar effective temperature of 5,772 K,[7] its radius can be calculated using the Stefan-Boltzmann law.[lower-alpha 1]

Westerlund 1-20 was observed to have an extended, cometary shaped nebula, similar to the other red supergiant Westerlund 1-26. It is therefore likely that its morphology was affected by either the intracluster medium or the cluster wind of Westerlund 1.[8] The nebulae of both Westerlund 1-20 and Westerlund 1-26 are extended outward from the cluster core and most bright at inward direction, indicating the outward cluster wind.[9]

See also

Notes

References
  1. Fok, 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. S2CID 53393926.
  2. Arévalo, Aura (2019). The Red Supergiants in the Supermassive Stellar Cluster Westerlund 1 (Thesis). doi:10.11606/D.14.2019.tde-12092018-161841.
  3. Westerlund, B. E. (1987). "Photometry and spectroscopy of stars in the region of a highly reddened cluster in Ara". Astronomy and Astrophysics Supplement Series. 70: 311. Bibcode:1987A&AS...70..311W.
  4. Brown, A. G. A.; et al. (Gaia collaboration). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics (in press). arXiv:2012.01533. doi:10.1051/0004-6361/202039657. S2CID 227254300. Gaia EDR3 record for this source at VizieR.
  5. Aghakhanloo, Mojgan; Murphy, Jeremiah W.; Smith, Nathan; Parejko, John; Díaz-Rodríguez, Mariangelly; Drout, Maria R.; Groh, Jose H.; Guzman, Joseph; Stassun, Keivan G. (2020-02-21). "Inferring the parallax of Westerlund 1 from Gaia DR2". Monthly Notices of the Royal Astronomical Society. 492 (2): 2497–2509. arXiv:1901.06582. Bibcode:2020MNRAS.492.2497A. doi:10.1093/mnras/stz3628. ISSN 0035-8711. S2CID 119465620.
  6. "Cl* Westerlund 1 W 20". Simbad.
  7. Williams, D. R. (2013). "Sun Fact Sheet".
  8. Dougherty, S. M.; Clark, J. S.; Negueruela, I.; Johnson, T.; Chapman, J. M. (2010-02-01). "Radio emission from the massive stars in the galactic super star cluster Westerlund 1". Astronomy & Astrophysics. 511: A58. arXiv:0912.4165. Bibcode:2010A&A...511A..58D. doi:10.1051/0004-6361/200913505. ISSN 0004-6361.
  9. Asymmetric Ejecta of Cool Supergiants and Hypergiants in the Massive Cluster Westerlund 1, 2018, arXiv:1803.07008
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