| Observation data EpochJ2000.0 EquinoxJ2000.0 (ICRS) | |
|---|---|
| Constellation | Hydrus |
| Right ascension | 03h 02m 15.449s[1] |
| Declination | −71° 54′ 08.84″[1] |
| Apparent magnitude (V) | 5.53[2] |
| Characteristics | |
| Spectral type | B8 III/IV[3] |
| U−Bcolor index | −0.51[2] |
| B−Vcolor index | −0.14[2] |
| Astrometry | |
| Radial velocity (Rv) | +12.3±1.6[4] km/s |
| Proper motion (μ) | RA: +27.256mas/yr[1] Dec.: +16.598mas/yr[1] |
| Parallax (π) | 6.4996±0.0619 mas[1] |
| Distance | 502 ± 5 ly (154 ± 1 pc) |
| Absolute magnitude (MV) | −0.48[5] |
| Details | |
| Luminosity | 287[5] L☉ |
| Surface gravity (log g) | 3.76[6] cgs |
| Temperature | 13,350[6] K |
| Rotation | 4.3720[7]d |
| Age | 211[8] Myr |
| Other designations | |
| θ Hyi,CPD−72°219,FK5 113,HD 19400,HIP 14131,HR 939,SAO 255945,WDS J03023-7154AB[9] | |
| Database references | |
| SIMBAD | data |
Theta Hydri,Latinized from θ Hydri, is a blue-white huedstar in the southernconstellation ofHydrus. It is faintly visible to the naked eye with anapparent visual magnitude of +5.53.[2] Based upon an annualparallax shift of6.50 mas as seen from Earth,[1] it is located approximately502 light years. At that distance, the visual magnitude of the star is diminished by anextinction of 0.10 due tointerstellar dust.[8] It is moving away from the Sun with aradial velocity of+12.3 km/s.[4]
Astellar classification of B8 III/IV[3] suggests it is anevolvingB-type star showing mixed traits of asubgiant orgiant star. It is a PGa star – a sub-class of the higher temperaturechemically peculiar stars known asmercury-manganese stars (HgMn stars). That is, it displays a richspectra of singly-ionized phosphorus and gallium, in addition to ionized mercury and manganese. As such, Theta Hydri forms a typical example of this type. Theabsorption lines for these ionized elements are found to vary, most likely as the result of uneven surface distribution combined with the star's rotation.[10] It is ahelium-weak star, having heliumlines that are anomalously weak for its spectral type.[6] A weak and variable longitudinalmagnetic field has been detected.[10]
There is a nearby companion star of class A0 IV[11] located at anangular separation of0.1 arc seconds along aposition angle of 179°, as of 2002. Schöller et al. (2010) consider this to be a visual companion,[12] although Eggleton and Tokovinin (2008) listed the pair as a probablebinary star system.[11]
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