tau1 Hydrae

{{Short description|Triple star system in the constellation Hydra}}

{{other stars by Bayer designation|Tau Hydrae{{!}}τ Hydrae}}

{{Starbox begin

| name = Tau¹ Hydrae

}}

{{Starbox observe

| epoch = J2000.0

| equinox = J2000.0 (ICRS)

| constell = Hydra

| ra = {{RA|09|29|08.89655}}

| dec = {{DEC|-02|46|08.2649}}

| appmag_v = +4.59 {{nowrap|(4.60 + 7.15)}}

}}

{{Starbox character

| class = {{nowrap|F6 V + ? + K0}}

| b-v = {{Val|+0.411|0.015}}

| u-b =

| r-i =

| variable =

}}

{{Starbox astrometry

| radial_v = {{Val|+10.85|0.28}}

| absmag_v = +3.28

}}

{{Starbox astrometry|no_heading=y

| component1 = τ¹ Hydrae A

| prop_mo_ra = +107.115

| prop_mo_dec = −29.652

| parallax = 56.2938

| p_error = 0.5309

| parallax_footnote =

| component2 = τ¹ Hydrae B

| prop_mo_ra2 = +138.487

| prop_mo_dec2 = −17.371

| parallax2 = 55.3675

| p_error2 = 0.0638

| parallax_footnote2 =

}}

{{Starbox orbit

| reference =

| period_unitless = {{Val|2807|23|fmt=commas|u=d}}

| axis =

| axis_unitless =

| eccentricity = {{Val|0.33|0.12}}

| inclination =

| node =

| periastron = {{nowrap|2445260 ± 150 JD}}

| periarg =

| k1 = {{Val|2.98|0.39}}

}}

{{Starbox detail

| component1 = τ¹ Hydrae A

| mass = 1.20

| radius = 1.4

| luminosity_bolometric = 3.369

| temperature = {{Val|6473|220|fmt=commas}}

| gravity = {{Val|4.12|0.14}}

| metal_fe = −0.01

| rotational_velocity = {{Val|30.4|1.5}}

| age_gyr = 3.61

| component2 = τ¹ Hydrae B

| mass2 = 0.86

| radius2 = 0.81

| luminosity2 = 0.435

| temperature2 = 5,197

}}

{{Starbox catalog

| names = τ¹ Hya, 31 Hydrae, BD−02°2901, GJ 348, HD 81997, HIP 46509, HR 3759, SAO 136895, WDS J09291-0246

}}

{{Starbox reference

| Simbad = **+HJ+1167 | sn=τ¹ Hya AB

| Simbad2 = 31+Hya | sn2=τ¹ Hya A

| Simbad3 = *+31+Hya+B | sn3=τ¹ Hya B

}}

Tau¹ Hydrae is a triple star system in the equatorial constellation of Hydra. Based upon the annual parallax shift of the two visible components as seen from Earth, they are located about {{convert|18|pc|ly}} from the Sun. The system has a combined apparent visual magnitude of +4.59, 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. The visible member of the pair, component A, is a visual magnitude 4.60 F-type main sequence star with a stellar classification of F6 V. 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.

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

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