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Tau¹ Hydrae
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Hydra
Right ascension	09h 29m 08.89655s
Declination	−02° 46′ 08.2649″
Apparent magnitude (V)	+4.59 (4.60 + 7.15)
Characteristics
Spectral type	F6 V + ? + K0
B−V color index	+0.411±0.015
Astrometry
Radial velocity (Rv)	+10.85±0.28 km/s
Absolute magnitude (MV)	+3.28
τ1 Hydrae A
Proper motion (μ)	RA: +107.115 mas/yr ; Dec.: −29.652 mas/yr
Parallax (π)	56.2938 ± 0.5309 mas
Distance	57.9 ± 0.5 ly ; (17.8 ± 0.2 pc)
τ1 Hydrae B
Proper motion (μ)	RA: +138.487 mas/yr ; Dec.: −17.371 mas/yr
Parallax (π)	55.3675 ± 0.0638 mas
Distance	58.91 ± 0.07 ly ; (18.06 ± 0.02 pc)
Orbit
Period (P)	2,807±23 d
Eccentricity (e)	0.33±0.12
Periastron epoch (T)	2445260 ± 150 JD
Semi-amplitude (K1); (primary)	2.98±0.39 km/s
Details
τ1 Hydrae A
Mass	1.20 M☉
Radius	1.4 R☉
Luminosity (bolometric)	3.369 L☉
Surface gravity (log g)	4.12±0.14 cgs
Temperature	6,473±220 K
Metallicity [Fe/H]	−0.01 dex
Rotational velocity (v sin i)	30.4±1.5 km/s
Age	3.61 Gyr
τ1 Hydrae B
Mass	0.86 M☉
Radius	0.81 R☉
Luminosity	0.435 L☉
Temperature	5,197 K
Other designations
	τ1 Hya, 31 Hydrae, BD−02°2901, GJ 348, HD 81997, HIP 46509, HR 3759, SAO 136895, WDS J09291-0246
Database references
SIMBAD	τ1 Hya AB
	τ1 Hya A
	τ1 Hya B

Tau¹ Hydrae is a triple star^[3] system in the equatorial constellation of Hydra. Based upon the annual parallax shift of the two visible components as seen from Earth,^[1] they are located about 18 parsecs (59 ly) 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.^[8] 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.^[13]

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.^[14] As of 2012, it was positioned at an angular separation of 67.5 arc seconds along a position angle of 4°.^[15]

References

^ ^a ^b ^c van Leeuwen, F. (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, S2CID 18759600.
^ ^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, arXiv:1605.05837, Bibcode:2016A&A...593A..51M, doi:10.1051/0004-6361/201628329, S2CID 55251562, A51.
^ ^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:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
^ 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:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, S2CID 59451347, A61.
^ ^a ^b Reiners, Ansgar (January 2006), "Rotation- and temperature-dependence of stellar latitudinal differential rotation", Astronomy and Astrophysics, 446 (1): 267–277, arXiv:astro-ph/0509399, Bibcode:2006A&A...446..267R, doi:10.1051/0004-6361:20053911, S2CID 8642707
^ ^a ^b ^c Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ ^a ^b ^c ^d ^e Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ ^a ^b Pourbaix, D.; et al. (September 2004), "S_B⁹: The ninth catalogue of spectroscopic binary orbits", Astronomy and Astrophysics, 424: 727–732, arXiv:astro-ph/0406573, Bibcode:2004A&A...424..727P, doi:10.1051/0004-6361:20041213, S2CID 119387088.
^ ^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:1501.03154, Bibcode:2015ApJ...804..146D, doi:10.1088/0004-637X/804/2/146, S2CID 33401607.
^ ^a ^b Pasinetti Fracassini, L. E.; et al. (February 2001), "Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS)", Astronomy and Astrophysics, 367 (3rd ed.): 521–524, arXiv:astro-ph/0012289, Bibcode:2001A&A...367..521P, doi:10.1051/0004-6361:20000451, S2CID 425754.
^ Tokovinin, Andrei (2014), "From Binaries to Multiples. II. Hierarchical Multiplicity of F and G Dwarfs", The Astronomical Journal, 147 (4): 14, arXiv:1401.6827, Bibcode:2014AJ....147...87T, doi:10.1088/0004-6256/147/4/87, S2CID 56066740, 87.
^ "tau01 Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-03-22.{{cite web}}: CS1 maint: postscript (link)
^ Henry, G. W.; et al. (March 1999), "tau¹ Hydrae: NOT A gamma DORADUS VARIABLE", Information Bulletin on Variable Stars, 4688 (4688): 1, Bibcode:1999IBVS.4688....1H.
^ 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:1206.4289, Bibcode:2012AJ....144...62A, doi:10.1088/0004-6256/144/2/62, S2CID 51051184, 62.
^ Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122 (6): 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920.