Astronomy:Tau9 Eridani

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τ9 Eridani
Observation data
Epoch J2000.0 (ICRS)      Equinox J2000.0 (ICRS)
Constellation Eridanus
Right ascension  03h 59m 55.48381s[1]
Declination −24° 00′ 58.3798″[1]
Apparent magnitude (V) 4.63[2]
Spectral type B9.5V Si[3]
U−B color index −0.40[2]
B−V color index −0.13[2]
Variable type α2 CVn[4]
Radial velocity (Rv)25.5±0.5[5] km/s
Proper motion (μ) RA: +12.12[1] mas/yr
Dec.: +16.48[1] mas/yr
Parallax (π)9.96 ± 0.22[1] mas
Distance327 ± 7 ly
(100 ± 2 pc)
Absolute magnitude (MV)−0.44[4]
Period (P)5.9537 days
Eccentricity (e)0.1
Periastron epoch (T)2417600.95 JD
Argument of periastron (ω)
Semi-amplitude (K1)
37.6 km/s
τ9 Eri A
Mass3.26[7] M
Radius3.26[7] R
Luminosity166[8] L
Surface gravity (log g)4.11±0.11[9] cgs
Temperature10,866[8] K
Rotational velocity (v sin i)30[10] km/s
Other designations
τ9 Eridani, τ9 Eri, 36 Eridani, CD−24° 2022, FK5 2287, HD 25267, HIP 18673, HR 1240, SAO 169017.[11]
Database references

Tau9 Eridani (τ9 Eri) is a binary star in the constellation Eridanus. It is visible to the naked eye with an apparent visual magnitude of 4.63.[2] The distance to this system can be estimated using the parallax method, which yields a value of roughly 327 light years.[2]

This is a single-lined spectroscopic binary system with an orbital period of 5.9537 days and an eccentricity of 0.1.[6] The primary component, τ9 Eri A, is a magnetic chemically peculiar star[7] with a stellar classification of B9.5V Si,[3] indicating that it is a B-type main sequence star that shows abundance anomalies in its silicon absorption lines.[3] It is an Alpha² Canum Venaticorum variable[4] with a rotational periodicity of 5.954 days.[12] The averaged strength of the stellar effective magnetic field is 240.6±91.0 G.[3]

Tau9 Eridani A has an estimated 326%[7] the mass of the Sun and 3.1 times the Sun's radius.[9] It shines with 166[8] times the solar luminosity from a outer atmosphere at an effective temperature of 10,866 K.[8] The star is spinning with a projected rotational velocity of 30 km/s.[10]


  1. 1.0 1.1 1.2 1.3 1.4 van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, doi:10.1051/0004-6361:20078357, Bibcode2007A&A...474..653V. 
  2. 2.0 2.1 2.2 2.3 2.4 Mermilliod, J.-C. (1986), Compilation of Eggen's UBV data, transformed to UBV (unpublished), SIMBAD, Bibcode1986EgUBV........0M. 
  3. 3.0 3.1 3.2 3.3 Bychkov, V. D. et al. (April 2009), "Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar A and B stars", Monthly Notices of the Royal Astronomical Society 394 (3): 1338–1350, doi:10.1111/j.1365-2966.2008.14227.x, Bibcode2009MNRAS.394.1338B. 
  4. 4.0 4.1 4.2 Dubath, P. et al. (2011), "Random forest automated supervised classification of Hipparcos periodic variable stars", Monthly Notices of the Royal Astronomical Society 414 (3): 2602–2617, doi:10.1111/j.1365-2966.2011.18575.x, Bibcode2011MNRAS.414.2602D. 
  5. De Bruijne, J. H. J.; Eilers, A.-C. (2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics 546: A61, doi:10.1051/0004-6361/201219219, Bibcode2012A&A...546A..61D. 
  6. 6.0 6.1 Pourbaix, D. et al. (2004), "SB9: The ninth catalogue of spectroscopic binary orbits", Astronomy and Astrophysics 424 (2): 727–732, doi:10.1051/0004-6361:20041213, Bibcode2004A&A...424..727P. 
  7. 7.0 7.1 7.2 7.3 Shulyak, D. et al. (September 2014), "Interferometry of chemically peculiar stars: theoretical predictions versus modern observing facilities", Monthly Notices of the Royal Astronomical Society 443 (2): 1629–1642, doi:10.1093/mnras/stu1259, Bibcode2014MNRAS.443.1629S. 
  8. 8.0 8.1 8.2 8.3 McDonald, I. et al. (2012), "Fundamental Parameters and Infrared Excesses of Hipparcos Stars", Monthly Notices of the Royal Astronomical Society 427 (1): 343–57, doi:10.1111/j.1365-2966.2012.21873.x, Bibcode2012MNRAS.427..343M. 
  9. 9.0 9.1 North, P. (June 1998), "Do SI stars undergo any rotational braking?", Astronomy and Astrophysics 334: 181–187, Bibcode1998A&A...334..181N. 
  10. 10.0 10.1 Abt, Helmut A. et al. (July 2002), "Rotational Velocities of B Stars", The Astrophysical Journal 573 (1): 359–365, doi:10.1086/340590, Bibcode2002ApJ...573..359A. 
  11. "tau09 Eri". SIMBAD. Centre de données astronomiques de Strasbourg. 
  12. Bychkov, V. D. et al. (February 2005), "A catalog of stellar magnetic rotational phase curves", Astronomy and Astrophysics 430: 1143–1154, doi:10.1051/0004-6361:20034563, Bibcode2005A&A...430.1143B. Eridani was the original source. Read more.