# Epsilon Sagittarii

Observation data Epoch J2000      Equinox J2000 Constellation Location of ε Sgr (circled) Sagittarius 18h 24m 10.31840s[1] –34° 23′ 04.6193″[1] +1.85[2] B9.5 III[3] +0.13[2] –0.03[2] Radial velocity (Rv) –15[4] km/s Proper motion (μ) RA: –39.42[1] mas/yr Dec.: –124.20[1] mas/yr Parallax (π) 22.76 ± 0.24[1] mas Distance 143 ± 2 ly (43.9 ± 0.5 pc) Absolute magnitude (MV) –1.41[5] Mass 3.515 ± 0.138[5] M☉ Radius 6.8[6] R☉ Luminosity 363[5] L☉ Surface gravity (log g) 4.50[7] cgs Temperature 9,960[8] K Rotational velocity (v sin i) 236[9] km/s Age 232[5] Myr Mass 0.95[5] M☉ Luminosity 0.89[5] L☉ Temperature 5,807[5] K Kaus Australis, ε Sagittarii, ε Sgr, Epsilon Sgr, 20 Sagittarii, CCDM J18242-3423A, FK5 689, GC 25100, HD 169022, HIP 90185, HR 6879, IDS 18175-3427 A, PPM 297655, SAO 210091, WDS J18242-3423A SIMBAD data

Epsilon Sagittarii (ε Sagittarii, abbreviated Epsilon Sgr, ε Sgr), also named Kaus Australis,[10] is a binary star system in the southern zodiac constellation of Sagittarius. The apparent visual magnitude of +1.85[2] makes it the brightest star in the constellation. Based upon parallax measurements, this star is around 143 light-years (44 parsecs) from the Sun.

## Properties

The primary component of this binary star system has a stellar classification of B9.5 III,[3] with the luminosity class of III suggesting this is an evolved giant star that has exhausted the supply of hydrogen at its core. The interferometry-measured angular diameter of this star, after correcting for limb darkening, is 1.44 ± 0.06 mas,[11] which, at its estimated distance, equates to a physical radius of about 6.8 times the radius of the Sun.[6] This is a close match to the empirically-determined value of 6.9 solar radii.[12] It has about 3.5 times the mass of the Sun and is radiating around 363 times the Sun's luminosity from its outer atmosphere at an effective temperature of 9,960 K.[5] At this heat, the star glows with a blue-white hue.[13]

This star is spinning rapidly with a projected rotational velocity of 236 km s−1.[9] It has a magnetic field with a strength in the range 10.5–130.5 G[14] and it is an X-ray source with a luminosity of about 1030 erg s−1.[5] The system displays an excess emission of infrared radiation, which suggests the presence of a circumstellar disk of dust. Based upon the temperature of this disk, it is orbiting at a mean separation of 155 AU from the primary.[15]

As of 2001, the secondary companion is located at an angular separation of 2.392 arcseconds from the primary along a position angle of 142.3°. At the distance of this system, this angle is equivalent to a physical separation of about 106 AU, which places it inside the debris disk. It is a main sequence star with about 95% of the mass of the Sun. The system has a higher optical linear polarisation than expected for its distance from the Sun; this has been attributed to light scattered off the disk from the secondary.[16] Prior to its 1993 identification using an adaptive optics coronagraph, this companion may have been responsible for the spectral anomalies that were attributed to the primary star.[17] There is a candidate stellar companion at an angular separation of 32.3 arcseconds.[5]

## Nomenclature

ε Sagittarii (Latinised to Epsilon Sagittarii) is the star's Bayer designation.

It bore the traditional name Kaus Australis, which derived from the Arabic قوس qaws 'bow' and Latin austrālis 'southern'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[18] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[19] included a table of the first two batches of names approved by the WGSN; which included Kaus Australis for this star.

In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Thalath al Waridah, or Thalith al Waridah, meaning 'third of Warida'.[20]

In Chinese, (), meaning Winnowing Basket, refers to an asterism consisting of Epsilon Sagittarii, Gamma Sagittarii, Delta Sagittarii and Eta Sagittarii. Consequently, Epsilon Sagittarii itself is known as 箕宿三 (Jī Sù sān, English: the Third Star of Winnowing Basket.)[21]

This star, together with :

Kaus Australis is listed in the Babylonian compendium MUL.APIN as MA.GUR8, meaning "the Bark".[24]

The Kalapalo people of Mato Grosso state in Brazil called this star and λ Sco, through ι Sco, θ Sco, ν Sco, υ Sco and ρ Sco Taugi kusugu, "Taugi's fishing basket".[25]

## References

1. van Leeuwen, F. (November 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. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4: 99–100, Bibcode:1966CoLPL...4...99J
3. ^ a b Houk, Nancy; Smith-Moore, M. (1979), "Michigan catalogue of two-dimensional spectral types for the HD stars", Michigan Catalogue of Two-dimensional Spectral Types for the HD stars. Volume_3. Declinations -40_ƒ0 to -26_ƒ0, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, 3, Bibcode:1982mcts.book.....H
4. ^ Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick. Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. Determination of Radial Velocities and their Applications. 30. University of Toronto: International Astronomical Union. p. 57. Bibcode:1967IAUS...30...57E.
5. Hubrig, S.; Le Mignant, D.; North, P.; Krautter, J. (June 2001), "Search for low-mass PMS companions around X-ray selected late B stars", Astronomy and Astrophysics, 372: 152–164, arXiv:astro-ph/0103201, Bibcode:2001A&A...372..152H, doi:10.1051/0004-6361:20010452
6. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
{\displaystyle {\begin{aligned}2\cdot R_{*}&={\frac {(10^{-3}\cdot 43.9\cdot 1.44)\ {\text{AU}}}{0.0046491\ {\text{AU}}/R_{\bigodot }}}\\&\approx 13.6\cdot R_{\bigodot }\end{aligned}}}
7. ^ Adelman, Saul J. (December 2004). "The physical properties of normal A stars". In Zverko, J.; Ziznovsky, J.; Adelman, S. J.; Weiss, W. W. The A-Star Puzzle, held in Poprad, Slovakia, July 8-13, 2004. IAU Symposium. 2004 (224). Cambridge, UK: Cambridge University Press. pp. 1–11. Bibcode:2004IAUS..224....1A. doi:10.1017/S1743921304004314.
8. ^ Zorec, J.; et al. (July 2009), "Fundamental parameters of B supergiants from the BCD system. I. Calibration of the (λ_1, D) parameters into Teff", Astronomy and Astrophysics, 501 (1): 297–320, arXiv:0903.5134, Bibcode:2009A&A...501..297Z, doi:10.1051/0004-6361/200811147
9. ^ a b Royer, F.; et al. (October 2002), "Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i", Astronomy and Astrophysics, 393 (3): 897–911, arXiv:astro-ph/0205255, Bibcode:2002A&A...393..897R, doi:10.1051/0004-6361:20020943
10. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
11. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431 (2): 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039
12. ^ Jerzykiewicz, M.; Molenda-Zakowicz, J. (September 2000), "Empirical Luminosities and Radii of Early-Type Stars after Hipparcos", Acta Astronomica, 50: 369–380, Bibcode:2000AcA....50..369J
13. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
14. ^ Bychkov, V. D.; Bychkova, L. V.; Madej, J. (August 2003), "Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars", Astronomy and Astrophysics, 407 (2): 631–642, arXiv:astro-ph/0307356, Bibcode:2003A&A...407..631B, doi:10.1051/0004-6361:20030741
15. ^ Rodriguez, David R.; Zuckerman, B. (February 2012), "Binaries among Debris Disk Stars", The Astrophysical Journal, 745 (2): 147, arXiv:1111.5618, Bibcode:2012ApJ...745..147R, doi:10.1088/0004-637X/745/2/147
16. ^ Cotton, D. V.; et al. (January 2016). "The linear polarization of Southern bright stars measured at the parts-per-million level". Monthly Notices of the Royal Astronomical Society. 455 (2): 1607–1628. arXiv:1509.07221. Bibcode:2016MNRAS.455.1607C. doi:10.1093/mnras/stv2185.
17. ^ Golimowski, David A.; Durrance, Samuel T.; Clampin, Mark (March 1993), "Detection of an apparent star 2.1 arcsec from the circumstellar disk candidate Epsilon Sagittarii", Astronomical Journal, 105 (3): 1108–1113, Bibcode:1993AJ....105.1108G, doi:10.1086/116498
18. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
19. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
20. ^ Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55 (8): 435. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
21. ^
22. ^ "Teapot". constellation-guide.com. Retrieved 2017-05-13.
23. ^ a b Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 355. ISBN 0-486-21079-0. Retrieved 2012-09-04.
24. ^ Rogers, J. H. (February 1998), "Origins of the ancient constellations: I. The Mesopotamian traditions", Journal of the British Astronomical Association, 108 (1): 9–28, Bibcode:1998JBAA..108....9R
25. ^ Basso, Ellen B. (1987). In Favor of Deceit: A Study of Tricksters in an Amazonian Society. Tucson, Arizona: University of Arizona Press. p. 360. ISBN 0-8165-1022-9.