Epsilon Sagittarii
 Epsilon Sagittarii (circled) in the constellation Sagittarius. |
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| Observation data Epoch J2000 Equinox J2000 |
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|---|---|
| Constellation | Sagittarius |
| Right ascension | 18h 24m 10.31840s[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] |
| 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 | |
Epsilon Sagittarii (ε Sgr, ε Sagittarii) is a binary star system in the southern zodiac constellation Sagittarius. The star forms the base of the archer's "bow" in Sagittarius and also marks the base of the spout of the so-called "Teapot" asterism. Its traditional name is Kaus Australis, from Arabic قوس qaws 'bow' and Latin austrālis 'southern'. 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).
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[edit] 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]
[edit] Name
The traditional name of this star is Kaus Australis, from Arabic قوس qaws 'bow' and Latin austrālis 'southern'. The traditional Chinese name for this star is 箕宿三 (Mandarin: qí xìu sān), the Third Star of the Winnowing Basket. Kaus Australis is listed in MUL.APIN as MA.GUR8, meaning "the Bark".[17]
[edit] References
- ^ a b c d e van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, Bibcode 2007A&A...474..653V, doi:10.1051/0004-6361:20078357
- ^ 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
- ^ 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
- ^ 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
- ^ a b c d e f g h i j 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
- ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 1 (3 ed.), Birkhäuser, ISBN 3540296921, http://books.google.com/books?id=OvTjLcQ4MCQC&pg=PA41. The radius (R*) is given by:
- ^ 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): pp. 1–11, Bibcode 2004IAUS..224....1A, doi:10.1017/S1743921304004314
- ^ 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
- ^ 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
- ^ "KAUS AUSTRALIS -- Star in double system", SIMBAD (Centre de Données astronomiques de Strasbourg), http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Epsilon+Sagittarii, retrieved 2012-02-22
- ^ 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
- ^ 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
- ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html, retrieved 2012-01-16
- ^ 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
- ^ Rodriguez, David R.; Zuckerman, B. (February 2012), "Binaries among Debris Disk Stars", The Astrophysical Journal 745 (2): 147, Bibcode 2012ApJ...745..147R, doi:10.1088/0004-637X/745/2/147
- ^ 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
- ^ 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
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