KW Sagittarii

KW Sagittarii is a red supergiant, located approximately 2,400 parsecs away from our Sun in the direction of the constellation Sagittarius. It has a size over 1,000 R☉ making it one of the largest known stars. If placed at the center of the Solar System, the star's surface would engulf Mars.

KW Sagittarii
Location of KW Sgr
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]

8.5 to 11 (AAVSO)
11.0 to 13.2 (p)[3]

Characteristics
Spectral type M1.5Iab[4] (M0I - M4Ia[3])
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[3]
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
Distanceapprox. 6,000 ly
(approx. 1,900 pc)
Details
Radius1,009±142[4] R☉
Luminosity (bolometric)176,000+164,000
−87,000[4] L☉
Surface gravity (log g)0.0[4] cgs
Temperature3,720±183[4] K
Other designations
KW Sgr, CD−27°12032, HD 316496, HIP 87433, AAVSO 1745-28
Database references
SIMBADdata

Distance

The distance of 2,400 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]

Characteristics

In 2005, Levesque calculate that the star has a bolometric luminosity of over 360,000 L☉ and a radius around 1,460 R☉ based on the assumption of an effective temperature of 3,700 K.[8] In 2012, Marcaide calculate that the star has a bolometric luminosity of less than 200,000 L☉ based on the measured flux and an assumed distance, and a radius around 1,009 ± 142 R☉ based on the measured angular diameter and luminosity. The effective temperature was then derived from the luminosity and radius.[4]

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

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.
  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. 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)
  4. 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.
  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. Mel'Nik, A. M.; Dambis, A. K. (2009). "Kinematics of OB-associations and the new reduction of the Hipparcos data". Monthly Notices of the Royal Astronomical Society. 400 (1): 518. arXiv:0909.0618. Bibcode:2009MNRAS.400..518M. doi:10.1111/j.1365-2966.2009.15484.x.
  8. 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.
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