Tau1 Hydrae
Triple star system in the constellation Hydra
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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.
 | |
| 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] |
| Characteristics | |
| Spectral type | F6 V + ? + K0[3] |
| B−V color index | +0.411±0.015[2] |
| Astrometry | |
| Radial velocity (Rv) | +10.85±0.28[4] km/s |
| Absolute magnitude (MV) | +3.28[5] |
| τ1 Hydrae A | |
| Proper motion (μ) | RA: +107.115[6] mas/yr Dec.: −29.652[6] mas/yr |
| Parallax (π) | 56.2938±0.5309 mas[6] |
| Distance | 57.9 ± 0.5 ly (17.8 ± 0.2 pc) |
| τ1 Hydrae B | |
| Proper motion (μ) | RA: +138.487[7] mas/yr Dec.: −17.371[7] mas/yr |
| Parallax (π) | 55.3675±0.0638 mas[7] |
| Distance | 58.91 ± 0.07 ly (18.06 ± 0.02 pc) |
| Orbit[8] | |
| 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[9] M☉ |
| Radius | 1.4[10] R☉ |
| Luminosity (bolometric) | 3.369[2] L☉ |
| Surface gravity (log g) | 4.12±0.14[9] cgs |
| Temperature | 6,473±220[9] 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[11] M☉ |
| Radius | 0.81[10] R☉ |
| Luminosity | 0.435[7] L☉ |
| Temperature | 5,197[7] K |
| Other designations | |
| τ1 Hya, 31 Hydrae, BD−02°2901, GJ 348, HD 81997, HIP 46509, HR 3759, SAO 136895, WDS J09291-0246[12] | |
| Database references | |
| SIMBAD | τ1 Hya AB |
| τ1 Hya A | |
| τ1 Hya B | |
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]
This star along with τ2 Hydrae, ι Hydrae and 33 Hydrae (A Hydrae) were traditionally known as Ukdah, Arabic for "knot".[16] The name Ukdah is now officially applied to ι Hydrae.