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
Distance470 ± 40 ly
(150 ± 10 pc)
Absolute magnitude (MV)−4.2[5]
Mass15.0 ± 0.1,[6] 14.5–14.7[7] M
Radius6.5[3] R
Diameter6.5[3] D
Luminosity18,000,[8] 20,400[7] L
Surface gravity (log g)4.24[9] cgs
Temperature31,440,[10] 29,850[7] K
Metallicity [Fe/H]−0.14[9] dex
Rotation41 days[11]
Rotational velocity (v sin i)24[12] km/s
Age5.7 ± 1.0,[6] 5[7] Myr
Other designations
Paikauhale (star A), Alniyat, Al Niyat τ Sco, 23 Sco, FK5 620, GC 22303, HD 149438, HIP 81266, HR 6165, SAO 184481.[13]
Database references

Tau Scorpii (τ Sco, τ Scorpii) is a star in the southern zodiac constellation of Scorpius. 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 20,400[7] 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 kelvins 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.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 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. 2.0 2.1 2.2 2.3 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. 3.0 3.1 3.2 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/9912360, 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, Astronomisches Rechen-Institut Heidelberg, Bibcode 1999VeARI..35....1W
  5. 5.0 5.1 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. 6.0 6.1 6.2 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:1007.4883, Bibcode 2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 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:1112.1695. Bibcode 2012ApJ...746..154P. doi:10.1088/0004-637X/746/2/154.
  8. Kaler, James B., Al NIYAT (Tau Scorpii), University of Illinois, http://stars.astro.illinois.edu/sow/alniyat-t.html, retrieved 2010-08-01
  9. 9.0 9.1 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. 10.0 10.1 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
  11. 11.0 11.1 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), http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Tau+Sco, retrieved 2010-08-01
  14. "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
  15. 15.0 15.1 15.2 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, http://dare.uva.nl/document/40494
  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/0606156, Bibcode 2006MNRAS.370..629D, doi:10.1111/j.1365-2966.2006.10558.x