# Epsilon Sagittarii

Observation data Characteristics Epoch J2000      Equinox J2000 Epsilon Sagittarii (circled) in the constellation Sagittarius. Constellation Sagittarius Right ascension 18h 24m 10.31840s[1] Declination –34° 23′ 04.6193″[1] Apparent magnitude (V) +1.85[2] Spectral type B9.5 III[3] U−B color index +0.13[2] B−V color index –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.[10]

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

## 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. 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.[16] There is a candidate stellar companion at an angular separation of 32.3 arcseconds.[5]

## 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 3, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, 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 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 (3 ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R*) is given by:
\begin{align} 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{align}
7. ^ Adelman, Saul J. (December 2004), The physical properties of normal A stars, in Zverko, J.; Ziznovsky, J.; Adelman, S. J. et al., "The A-Star Puzzle, held in Poprad, Slovakia, July 8-13, 2004", IAU Symposium (Cambridge, UK: Cambridge University Press) (224): 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, 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. ^ "KAUS AUSTRALIS -- Star in double system", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2012-02-22
11. ^ Richichi, A.; Percheron, I.; Khristoforova, M. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics 431: 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, 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: 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. ^ 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
17. ^ "Sagittarius". deepsky.astroinfo.org. Retrieved 30 August 2008.
18. ^
19. ^ 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.
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: 435. Bibcode:1895MNRAS..55..429K.
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22. ^ 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