Astronomy:KW Sagittarii

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Short description: Red supergiant star in the constellation Sagittarius
KW Sagittarii
Observation data
Equinox J2000.0]] (ICRS)
Constellation Sagittarius
Right ascension  17h 52m 00.72695s[1]
Declination −28° 01′ 20.5557″[1]
Apparent magnitude (V) 11.0[2]
Characteristics
Evolutionary stage Red supergiant or hypergiant[3]
Spectral type M1.5Iab[4] (M0I - M4Ia[5])
Apparent magnitude (K) 1.43[2]
U−B color index 3.21[4]
B−V color index 2.47[4]
V−R color index 2.58[4]
J−K color index 1.56[4]
Variable type SRc[5]
Astrometry
Radial velocity (Rv)−7.40[6] km/s
Proper motion (μ) RA: +0.843[1] mas/yr
Dec.: −0.946[1] mas/yr
Parallax (π)0.4355 ± 0.0726[1] mas
Distance7,890 ly
(2,420 pc)
Absolute magnitude (MV)−7.7[7]
Details
Mass20 (or 40[lower-alpha 1])[4] M☉
Radius1,009±142[4] R☉
Luminosity (bolometric)175,000±52,000[4] L☉
Surface gravity (log g)0.0[4] cgs
Temperature3,720±183[4] K
Other designations
KW Sgr, CD−27°12032, Gaia DR2 4063462206570029312, HD 316496, HIP 87433
Database references
SIMBADdata

KW Sagittarii is a red supergiant or hypergiant[3] star, located approximately 2,420 parsecs (7,900 light-years) away from the Sun in the direction of the constellation Sagittarius. It is one of the largest known stars, with a diameter about 1,000 times larger than the Sun. If placed at the center of the Solar System, the star's surface would engulf Mars, coming close to Jupiter's orbit.

Variability

AAVSO light curve of KW Sgr from 1 January 1990 to 24 November 2010. Up is brighter and down is fainter. Day numbers are Julian day.

In 1942, Henrietta Hill Swope listed KW Sagittarii as a variable star.[8] It varies erratically in brightness over a range of about two magnitudes.[9] It is classified as a semiregular variable,[5] although the listed period of 670 days is poorly defined.[9] The peculiar cool spectrum has led to comparisons with symbiotic variables, but it is no longer considered to be a cataclysmic binary.[10]

Distance

A distance of 2,420 parsecs is based on the assumption of membership on the Sagittarius OB5 association.[11] The parallax derived from the Hipparcos mission is negative so doesn't give much information about the distance except that it is likely to be large.[12] The Gaia Data Release 2 parallax is 0.5281±0.1392 mas and implies a distance of around 1,900 pc. The Gaia result carries a significant statistical margin of error, as well as an indicator that the astrometric excess noise is far beyond acceptable levels so that the parallax should be considered unreliable.[13] A 2021 study published a photogeometric distance of 2,159 pc to KW Sgr, using a parallax published by Gaia DR3 (the successor of Gaia DR2).[14]

Characteristics

KW Sagittarii is classed as a dust-enshrouded luminous cool supergiant and varies its spectral type between M0 and M4.[5][15] Sometimes described as a cool hypergiant,[3] it is losing its mass at 5.6×10−6 M☉ per year and is also found to be a confirmed maser source of SiO masers.[16][17] H2O and OH masers around the star have been proposed, but neither have been detected.[17]

A 2005 study led by Levesque, using a MARCS model, calculated a high luminosity of 363,000 L☉ for KW Sgr and consequently very large radius of 1,460 R☉ based on the assumption of an effective temperature of 3,700 K at a distance of 3.0 kpc. The star was then described as among the four largest and most luminous galactic red supergiants, which includes V354 Cephei, KY Cygni and Mu Cephei.[18] In 2011, its bolometric luminosity was re-measured to be at around 228,000 L☉ based on the same distance.[16]

More recently, the Sagittarius OB5 association was calculated to be somewhat closer than previously thought, which would suggest lower estimates for both the radius and luminosity of the star.[4] Based on a distance of 2.4±0.3 kpc, its luminosity was calculated to be 175,000±52,000 L☉ with a radius of around 1,009±142 R☉ based on a measured angular diameter of 3.91±0.25 mas.[4] Its angular diameter was re-measured at 3.00±0.4 mas in 2023.[19]

See also

Notes

  1. ↑ Assuming that KW Sagittarii doesn't rotate.

References

  1. ↑ 1.0 1.1 1.2 1.3 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  2. ↑ 2.0 2.1 Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues 2237. Bibcode: 2002yCat.2237....0D. 
  3. ↑ 3.0 3.1 3.2 Jones, Terry (2025). "Red and Yellow Hypergiants". Galaxies 13 (2): 43. doi:10.3390/galaxies13020043. Bibcode: 2025Galax..13...43J. 
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 Arroyo-Torres, B.; Wittkowski, M.; Marcaide, J. M.; Hauschildt, P. H. (2013). "The atmospheric structure and fundamental parameters of the red supergiants AH Scorpii, UY Scuti, and KW Sagittarii". Astronomy & Astrophysics 554: A76. doi:10.1051/0004-6361/201220920. Bibcode: 2013A&A...554A..76A. 
  5. ↑ 5.0 5.1 5.2 5.3 KW Sgr, database entry, The combined table of GCVS Vols I-III and NL 67-78 with improved coordinates, General Catalogue of Variable Stars , Sternberg Astronomical Institute, Moscow, Russia. Accessed on line November 10, 2010. (Quick look: KW+Sgr)
  6. ↑ Barbier-Brossat, M.; Petit, M.; Figon, P. (1994). "Third bibliographic catalogue of stellar radial velocities (Text in French)". Astronomy and Astrophysics Supplement Series 108: 603. Bibcode: 1994A&AS..108..603B. 
  7. ↑ Massey, Philip; Silva, David R.; Levesque, Emily M.; Plez, Bertrand; Olsen, Knut A. G.; Clayton, Geoffrey C.; Meynet, Georges; Maeder, Andre (2009). "Red Supergiants in the Andromeda Galaxy (M31)". The Astrophysical Journal 703 (1): 420. doi:10.1088/0004-637X/703/1/420. Bibcode: 2009ApJ...703..420M. 
  8. ↑ Swope, Henrietta Hill (1942). "Variable stars in MWF 189". Annals of Harvard College Observatory 109: 1–10. https://articles.adsabs.harvard.edu/pdf/1942AnHar.109....1S. Retrieved 13 December 2024. 
  9. ↑ 9.0 9.1 Swope, Henrietta Hill (1942). "Variable stars in MWF 189". Annals of Harvard College Observatory 109: 1. Bibcode: 1942AnHar.109....1S. 
  10. ↑ Downes, Ronald A.; Webbink, Ronald F.; Shara, Michael M.; Ritter, Hans; Kolb, Ulrich; Duerbeck, Hilmar W. (2001). "A Catalog and Atlas of Cataclysmic Variables: The Living Edition". Publications of the Astronomical Society of the Pacific 113 (784): 764. doi:10.1086/320802. Bibcode: 2001PASP..113..764D. 
  11. ↑ Melnik, A. M.; Dambis, A. K. (2020). "Distance scale for high-luminosity stars in OB associations and in field with Gaia DR2. Spurious systematic motions". Astrophysics and Space Science 365 (7): 112. doi:10.1007/s10509-020-03827-0. Bibcode: 2020Ap&SS.365..112M. 
  12. ↑ Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. doi:10.1051/0004-6361:20078357. Bibcode: 2007A&A...474..653V. 
  13. ↑ Brown, A. G. A. (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics 616: A1. doi:10.1051/0004-6361/201833051. Bibcode: 2018A&A...616A...1G.  Gaia DR2 record for this source at VizieR.
  14. ↑ Bailer-Jones, C. A. L.; Rybizki, J.; Fouesneau, M.; Demleitner, M.; Andrae, R. (2021-03-01). "Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3". The Astronomical Journal 161 (3): 147. doi:10.3847/1538-3881/abd806. ISSN 0004-6256. Bibcode: 2021AJ....161..147B.  Data about this star can be seen here.
  15. ↑ Van Loon, J. Th.; Cioni, M.-R. L.; Zijlstra, A. A.; Loup, C. (2005). "An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars". Astronomy and Astrophysics 438 (1): 273–289. doi:10.1051/0004-6361:20042555. Bibcode: 2005A&A...438..273V. 
  16. ↑ 16.0 16.1 Mauron, N.; Josselin, E. (2011). "The mass-loss rates of red supergiants and the de Jager prescription". Astronomy and Astrophysics 526: A156. doi:10.1051/0004-6361/201013993. Bibcode: 2011A&A...526A.156M. 
  17. ↑ 17.0 17.1 "Object G1.506-0.733". https://maserdb.net/object.pl?object=G1.506-0.733. 
  18. ↑ Levesque, E. M.; Massey, P.; Olsen, K. A. G.; Plez, B.; Josselin, E.; Maeder, A.; Meynet, G. (2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought". The Astrophysical Journal 628 (2): 973–985. doi:10.1086/430901. Bibcode: 2005ApJ...628..973L. 
  19. ↑ González-Torà, G.; Wittkowski, M.; Davies, B.; Plez, B. (December 19, 2023). "The effect of winds on atmospheric layers of red supergiants II. Modelling VLTI/GRAVITY and MATISSE observations of AH Sco, KW Sgr, V602 Car, CK Car and V460 Car". Astronomy & Astrophysics 683: A19. doi:10.1051/0004-6361/202348047. ISSN 0004-6361. Bibcode: 2024A&A...683A..19G. 



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