Epsilon Sagittarii

Epsilon Sagittarii

Location of ε CygSgr (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	18^h 24^m 10.31840^s[1]
Declination	–34° 23′ 04.6193″[1]
Apparent magnitude (V)	+1.85[2]
Characteristics
Spectral type	B9.5 III[3]
U−B color index	+0.13[2]
B−V color index	–0.03[2]
Astrometry
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]
Details
ε Sgr A
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
ε Sgr B
Mass	0.95[5] M☉
Luminosity	0.89[5] L☉
Temperature	5,807[5] K
Other designations
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
Database references
SIMBAD	data

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,^[10] 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.^[11] 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.^[12]

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^[13] 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.^[14]

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.^[15] 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]

Name and etymology

• The traditional name of this star is Kaus Australis, from Arabic قوس qaws 'bow' and Latin austrālis 'southern'.
• This star, together with :
  • γ Sgr, δ Sgr, ζ Sgr, λ Sgr, σ Sgr, τ Sgr and φ Sgr, comprising the asterism Teapot.^[17][18]
  • γ Sgr, δ Sgr and η Sgr were Al Naʽām al Wārid (النعم الوارد), the Going Ostriches.^[19]
  • γ Sgr and δ Sgr were Akkadian Sin-nun‑tu, or Si-nu-nu‑tum, the Swallow.^[19]
• 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, 箕 (Jī), meaning Winnowing Basket, refers to an asterism consisting of ε Sagittarii, γ Sagittarii, δ Sagittarii and η Sagittarii. Consequently, ε Sagittarii itself is known as 箕宿三 (Jī Sù sān, English: the Third Star of Winnowing Basket.)^[21]
• Kaus Australis is listed in MUL.APIN as MA.GUR₈, meaning "the Bark".^[22]

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. 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. 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. 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:

   $\begin{align} 2\cdot R_* & = \frac{(10^{-3}\cdot 43.9\cdot 1.44)\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\ & \approx 13.6\cdot R_{\bigodot} \end{align}$

7. Adelman, Saul J. (December 2004). "The physical properties of normal A stars". In Zverko, J.; Ziznovsky, J.; Adelman, S. J.; 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. 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. 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
11. 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
12. "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
13. 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
14. 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
15. 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.
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. skywatchers Archived May 17, 2007, at the Wayback Machine.
19. 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 (8): 435. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
21. (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
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

External links

• Kaus Australis