Tau1 Hydrae

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For other stars with this Bayer designation, see τ Hydrae.
Tau1 Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 09h 29m 08.89655s[1]
Declination −02° 46′ 08.2649″[1]
Apparent magnitude (V) +4.59 (4.60 + 7.15)[2]
Spectral type F6 V + ? + K0[3]
B−V color index +0.411±0.015[2]
Radial velocity (Rv) +10.85±0.28[4] km/s
Proper motion (μ) RA: +99.01[1] mas/yr
Dec.: −2.67[1] mas/yr
Parallax (π) 57.69 ± 2.14[1] mas
Distance 57 ± 2 ly
(17.3 ± 0.6 pc)
Absolute magnitude (MV) +3.28[5]
Period (P) 2,807±23 d
Eccentricity (e) 0.33±0.12
Periastron epoch (T) 2445260 ± 150 JD
Semi-amplitude (K1)
2.98±0.39 km/s
τ1 Hydrae A
Mass 1.20[7] M
Radius 1.4[8] R
Luminosity (bolometric) 3.369[2] L
Surface gravity (log g) 4.12±0.14[7] cgs
Temperature 6,473±220[7] K
Metallicity [Fe/H] −0.01[2] dex
Rotational velocity (v sin i) 30.4±1.5[5] km/s
Age 3.61[2] Gyr
τ1 Hydrae B
Mass 0.86[9] M
Radius 0.81[8] R
Other designations
Ukdah, τ1 Hya, 31 Hya, BD−02° 2901, GJ 348 A, HD 81997, HIP 46509, HR 3759, SAO 136895, WDS J09291-0246A.[10]
Database references
τ1 Hya A
τ1 Hya B

Tau1 Hydrae is a triple star[3] system in the equatorial constellation of Hydra. Based upon an annual parallax shift of 57.69 mas as seen from Earth,[1] it is located about 57 light years from the Sun. The system has a combined apparent visual magnitude of +4.59,[2] which is bright enough to be visible to the naked eye at night.

The inner pair of stars form a single-lined spectroscopic binary with an orbital period of about 2,807 days and an eccentricity of 0.33.[6] The visible member of the pair, component A, is a visual magnitude 4.60[2] F-type main sequence star with a stellar classification of F6 V.[3] During the 1990s, it was thought to be a Gamma Doradus variable, but this was later discounted as it shows no short-term photometric variability. The star does show some long-term variability, possibly as a result of a magnetic activity cycle similar to the solar cycle.[11]

The tertiary member, component B, is a visual magnitude 7.15[2] K-type star with a class of K0.[3] It lies at a separation of 1,120 AU from the primary.[12] As of 2012, it was positioned at an angular separation of 67.5 arc seconds along a position angle of 4°.[13]


  1. ^ a b c d e f van Leeuwen, F. (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 e f g h Montesinos, B.; et al. (September 2016), "Incidence of debris discs around FGK stars in the solar neighbourhood", Astronomy & Astrophysics, 593: 31, Bibcode:2016A&A...593A..51M, doi:10.1051/0004-6361/201628329, A51. 
  3. ^ a b c d Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878Freely accessible, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x. 
  4. ^ de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048Freely accessible, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, A61. 
  5. ^ a b Reiners, A. (January 2006), "Rotation- and temperature-dependence of stellar latitudinal differential rotation", Astronomy and Astrophysics, 446 (1): 267–277, arXiv:astro-ph/0509399Freely accessible, Bibcode:2006A&A...446..267R, doi:10.1051/0004-6361:20053911. 
  6. ^ a b Pourbaix, D.; et al. (September 2004), "SB9: The ninth catalogue of spectroscopic binary orbits", Astronomy and Astrophysics, 424: 727–732, arXiv:astro-ph/0406573Freely accessible, Bibcode:2004A&A...424..727P, doi:10.1051/0004-6361:20041213. 
  7. ^ a b c David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv:1501.03154Freely accessible, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146. 
  8. ^ a b Pasinetti Fracassini, L. E.; et al. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)", Astronomy and Astrophysics (3rd ed.), 367: 521–524, arXiv:astro-ph/0012289Freely accessible, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451. 
  9. ^ Tokovinin, Andrei (2014), "From Binaries to Multiples. II. Hierarchical Multiplicity of F and G Dwarfs", The Astronomical Journal, 147 (4): 14, arXiv:1401.6827Freely accessible, Bibcode:2014AJ....147...87T, doi:10.1088/0004-6256/147/4/87, 87. 
  10. ^ "tau01 Hya -- Spectroscopic binary", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-03-22. 
  11. ^ Henry, G. W.; et al. (March 1999), "tau1 Hydrae: NOT A gamma DORADUS VARIABLE", Information Bulletin on Variable Stars (4688): 1, Bibcode:1999IBVS.4688....1H. 
  12. ^ Allen, Peter R.; et al. (August 2012), "Low-mass Tertiary Companions to Spectroscopic Binaries. I. Common Proper Motion Survey for Wide Companions Using 2MASS", The Astronomical Journal, 144 (2): 12, arXiv:1206.4289Freely accessible, Bibcode:2012AJ....144...62A, doi:10.1088/0004-6256/144/2/62, 62. 
  13. ^ Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122: 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920.