KW Sagittarii

KW Sagittarii is a red supergiant, located approximately 1,900 parsecs away from our Sun in the direction of the constellation Sagittarius. It is one of the largest known stars. If placed at the center of the Solar System, the star's surface would engulf Mars.

KW Sagittarii
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Sagittarius
Right ascension 17h 52m 00.72665s[1]
Declination −28° 01′ 20.5622″[1]
Apparent magnitude (V) 11.0[2]
Characteristics
Spectral type M1.5Iab[3] (M0I - M4Ia[4])
Apparent magnitude (K) 1.43[2]
U−B color index 3.21[3]
B−V color index 2.47[3]
V−R color index 2.58[3]
J−K color index 1.56[3]
Variable type SRc[4]
Astrometry
Radial velocity (Rv)−7.40[5] km/s
Proper motion (μ) RA: 0.39[1] mas/yr
Dec.: −1.62[1] mas/yr
Parallax (π)0.5281 ± 0.1392[6] mas
Distance2,420[lower-alpha 1][7] pc
Absolute magnitude (MV)−7.7[8]
Details
Mass4.89+0.96
−0.07[9] M☉
Radius1,009±142[3] R☉
Luminosity (bolometric)176,000+164,000
−87,000[3] L☉
Surface gravity (log g)0.0[3] cgs
Temperature3,720±183[3] K
Other designations
KW Sgr, CD−27°12032, HD 316496, HIP 87433, AAVSO 1745-28
Database references
SIMBADdata

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

KW Sagitarii was discovered to be variable in 1928 from a study of photographic plates.[10] It varies erratically in brightness over a range of about two magnitudes.[11] It is classified as a semiregular variable,[4] although the listed period of 670 days is poorly-defined.[11] The peculiar cool spectrum has led to comparisons with symbiotic variables, but it is no longer considered to be a cataclysmic binary.[12]

Distance

A distance of 2,420 parsecs is based on the assumption of membership on the Sagittarius OB5 association.[7] 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.[1] The Gaia Data Release 2 parallax is 0.5281±0.1392 mas and implies a distance of around 1,900 pc. A calculation using a prior based on known galactic structure gives a distance of 1,945+1,039
−511 pc.[13] 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.[6]

Characteristics

KW Sagittarii is classed as a luminous cool supergiant and varies its spectral type between M0 and M4.[4] 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 3,000 pc. 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.[14]

More recently, KW Sagittarii was calculated to have a lower bolometric luminosity around 200,000 L☉ and a radius around 1,009 ± 142 R☉ was based on the measured angular diameter and luminosity.[3][15]

See also
  • VX Sagittarii - another red supergiant (RSG) in the constellation Sagittarius.

Notes
  1. Assumed as KW Sagittarii is assumed to be part of Sgr OB5 association

References
  1. Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. 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. 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. arXiv:1305.6179. Bibcode:2013A&A...554A..76A. doi:10.1051/0004-6361/201220920. S2CID 73575062.
  4. 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)
  5. 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.
  6. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  7. 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). arXiv:2006.14649. doi:10.1007/s10509-020-03827-0. S2CID 220128144.
  8. 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. arXiv:0907.3767. Bibcode:2009ApJ...703..420M. doi:10.1088/0004-637X/703/1/420. S2CID 119293010.
  9. Anders, F.; Khalatyan, A.; Chiappini, C.; Queiroz, A. B.; Santiago, B. X.; Jordi, C.; Girardi, L.; Brown, A. G. A.; Matijevič, G.; Monari, G.; Cantat-Gaudin, T.; Weiler, M.; Khan, S.; Miglio, A.; Carrillo, I.; Romero-Gómez, M.; Minchev, I.; De Jong, R. S.; Antoja, T.; Ramos, P.; Steinmetz, M.; Enke, H. (2019). "Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18". Astronomy and Astrophysics. 628: A94. arXiv:1904.11302. Bibcode:2019A&A...628A..94A. doi:10.1051/0004-6361/201935765. S2CID 131780028.
  10. Woods, Ida E. (1928). "Tenth Magnitude Nova in Sagittarius". Harvard College Observatory Bulletin. 861: 5. Bibcode:1928BHarO.861....5W.
  11. Swope, Henrietta Hill (1942). "Variable stars in MWF 189". Annals of Harvard College Observatory. 109: 1. Bibcode:1942AnHar.109....1S.
  12. 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. arXiv:astro-ph/0102302. Bibcode:2001PASP..113..764D. doi:10.1086/320802. S2CID 16285959.
  13. Bailer-Jones, C. A. L.; Rybizki, J.; Fouesneau, M.; Mantelet, G.; Andrae, R. (2018). "Estimating Distance from Parallaxes. IV. Distances to 1.33 Billion Stars in Gaia Data Release 2". The Astronomical Journal. 156 (2): 58. arXiv:1804.10121. Bibcode:2018AJ....156...58B. doi:10.3847/1538-3881/aacb21. S2CID 119289017.
  14. 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. arXiv:astro-ph/0504337. Bibcode:2005ApJ...628..973L. doi:10.1086/430901. S2CID 15109583.
  15. Mauron, N.; Josselin, E. (2011). "The mass-loss rates of red supergiants and the de Jager prescription". Astronomy and Astrophysics. 526: A156. arXiv:1010.5369. Bibcode:2011A&A...526A.156M. doi:10.1051/0004-6361/201013993. S2CID 119276502.
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