Tau Scorpii

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Tau Scorpii
Diagram showing star positions and boundaries of the Scorpius constellation and its surroundings
Cercle rouge 100%.svg

Location of τ Scorpii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Scorpius
Right ascension 16h 35m 52.95285s[1]
Declination −28° 12′ 57.6615″[1]
Apparent magnitude (V) +2.82[2]
Spectral type B0.2 V[3]
U−B color index −1.039[2]
B−V color index −0.252[2]
Radial velocity (Rv) +2.0[4] km/s
Proper motion (μ) RA: −9.89[1] mas/yr
Dec.: −22.83[1] mas/yr
Parallax (π) 6.88 ± 0.53[1] mas
Distance 470 ± 40 ly
(150 ± 10 pc)
Absolute magnitude (MV) −4.2[5]
Mass 15.0 ± 0.1,[6] [7] M
Radius 6.5[3] R
Luminosity 18,000,[8] 20,400[7] L
Surface gravity (log g) 4.24[9] cgs
Temperature 31,440,[10] 29,850[7] K
Metallicity [Fe/H] −0.14[9] dex
Rotation 41 days[11]
Rotational velocity (v sin i) 24[12] km/s
Age 5.7 ± 1.0,[6] 5[7] Myr
Other designations
Alniyat, Al Niyat τ Sco, 23 Sco, FK5 620, GC 22303, HD 149438, HIP 81266, HR 6165, SAO 184481.[13]

Tau Scorpii (τ Sco, τ Scorpii) is a star in the southern zodiac constellation of Scorpius. It has the traditional name Alniyat or Al Niyat, which it shares with σ Scorpii. The name derives from the Arabic النياط an-niyāţ meaning "the arteries". The apparent visual magnitude of Tau Scorpii is +2.8,[2] while parallax measurements yield a distance estimate of roughly 470 light-years (150 parsecs) from Earth.[1]

Surface magnetic field of Tau Scorpii as reconstructed by means of Zeeman–Doppler imaging

Compared to the Sun, Tau Scorpii is a massive OB star with 15[6] times the Sun's mass and more than six times the radius of the Sun.[3] It is radiating about 18,000[8] times the Sun's luminosity from its outer envelope at an effective temperature of 31,440 K.[10] This gives it the blue-white hue characteristic of B-type stars.[14] As yet there is no evidence of a companion in orbit around τ Sco.[15] It is a magnetic star whose surface magnetic field was mapped by means of Zeeman–Doppler imaging.[16] Tau Scorpii is rotating relatively slowly with a period of 41 days.[11]

The spectrum of this star shows triply ionized oxygen (O IV) that is being generated by X-rays and the Auger ionization effect. Observations with the ROSAT space telescope showed it has a higher energy (harder) X-ray spectrum than is usual for B0 V stars. Over the energy range 0.8–1.2 keV, its X-ray luminosity is Lx = 1.8 × 1031 erg s−1 with a large Lx to Lbol of log Lx/Lbol = –6.53 from ASCA measurements. ROSAT measurements showed a log Lx/Lbol ≃ –5.93 for the range 0.1-2.4 keV.[15] The hard component of the X-ray spectrum from τ Sco as studied with XMM-Newton supports the presence of in-falling clumps of plasma in τ Sco.[15]

This star is a proper motion member of the Upper Scorpius subgroup of the Scorpius-Centaurus OB association, the nearest such co-moving association of massive stars to the Sun.[5][7] The Upper Scorpius subgroup contains thousands of young stars with mean age 11 million years at average distance of 470 light years (145 parsecs)[7] A more recent analysis[7] of the HR diagram position for Tau Scorpii estimates its effective temperature to be 29,850 Kelvin with a luminosity of 20,400 Suns, consistent with an isochronal age of 5 million years and an estimated mass of 14.5-14.7 solar masses.

Culture signification[edit]

The indigenous Boorong people of northwestern Victoria saw this star (together with σ Sco) as wives of Djuit (Antares).[17]

See also[edit]


  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b c d Gutierrez-Moreno, Adelina; Moreno, Hugo (June 1968), "A Photometric Investigation of the Scorpio-Centaurus Association", Astrophysical Journal Supplement, 15: 459, Bibcode:1968ApJS...15..459G, doi:10.1086/190168 
  3. ^ a b c Howk, J. Christopher; et al. (May 2000), "Stagnation and Infall of Dense Clumps in the Stellar Wind of τ Scorpii", The Astrophysical Journal, 534 (1): 348–358, arXiv:astro-ph/9912360Freely accessible, Bibcode:2000ApJ...534..348H, doi:10.1086/308730 
  4. ^ Wielen, R.; et al. (1999), Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions (35), Astronomisches Rechen-Institut Heidelberg, Bibcode:1999VeARI..35....1W 
  5. ^ a b de Geus, E. J.; de Zeeuw, P. T.; Lub, J. (June 1989), "Physical parameters of stars in the Scorpio-Centaurus OB association", Astronomy and Astrophysics, 216 (1-2): 44–61, Bibcode:1989A&A...216...44D 
  6. ^ a b c Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883Freely accessible, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x 
  7. ^ a b c d e f g Mark J. Pecaut; et al. (February 2012). "A Revised Age for Upper Scorpius and the Star Formation History among the F-type Members of the Scorpius-Centaurus OB Association". Astrophysical Journal. 746 (2): 154. arXiv:1112.1695Freely accessible. Bibcode:2012ApJ...746..154P. doi:10.1088/0004-637X/746/2/154. 
  8. ^ a b Kaler, James B., Al NIYAT (Tau Scorpii), University of Illinois, retrieved 2010-08-01 
  9. ^ a b Kilian, J. (February 1994), "Chemical abundances in early B-type stars. 5: Metal abundances and LTE/NLTE comparison", Astronomy and Astrophysics, 282 (3): 867–873, Bibcode:1994A&A...282..867K 
  10. ^ a b 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:0903.5134Freely accessible, Bibcode:2009A&A...501..297Z, doi:10.1051/0004-6361/200811147 
  11. ^ a b Strassmeier, Klaus G. (September 2009), "Starspots", The Astronomy and Astrophysics Review, 17 (3): 251–308, Bibcode:2009A&ARv..17..251S, doi:10.1007/s00159-009-0020-6 
  12. ^ 6165, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID [1]. Accessed on line February 8, 2013.
  13. ^ "tau Sco -- Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2010-08-01 
  14. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16 
  15. ^ a b c Mewe, R.; et al. (2003), "High-resolution X-ray spectroscopy of τ Scorpii (B0.2V) with XMM-Newton", Astronomy & Astrophysics, 398 (1): 203–11, Bibcode:2003A&A...398..203M, doi:10.1051/0004-6361:20021577 
  16. ^ Donati, J.-F.; et al. (2006), "The surprising magnetic topology of τ Sco: fossil remnant or dynamo output?", Monthly Notices of the Royal Astronomical Society, 370 (2): 629–644, arXiv:astro-ph/0606156Freely accessible, Bibcode:2006MNRAS.370..629D, doi:10.1111/j.1365-2966.2006.10558.x 
  17. ^ Hamacher, Duane W.; et al. (2010). "An Aboriginal Australian Record of the Great Eruption of Eta Carinae" (PDF). Journal of Astronomical History & Heritage. 13 (3): 220–34. arXiv:1010.4610Freely accessible. Bibcode:2010JAHH...13..220H. 

Coordinates: Sky map 16h 35m 52.9537s, −28° 12′ 57.658″