Theta Hydri
Star in the constellation Hydrus
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Theta Hydri, Latinized from θ Hydri, is a blue-white hued star in the southern constellation of Hydrus. It is faintly visible to the naked eye with an apparent visual magnitude of +5.53.[2] Based upon an annual parallax shift of 6.50 mas as seen from Earth,[1] it is located approximately 502 light years. At that distance, the visual magnitude of the star is diminished by an extinction of 0.10 due to interstellar dust.[8] It is moving away from the Sun with a radial velocity of +12.3 km/s.[4]
| Observation data Epoch J2000.0 Equinox J2000.0 (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−B color index | −0.51[2] |
| B−V color index | −0.14[2] |
| Astrometry | |
| Radial velocity (Rv) | +12.3±1.6[4] km/s |
| Proper motion (μ) | RA: +27.256 mas/yr[1] Dec.: +16.598 mas/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 |
A stellar classification of B8 III/IV[3] suggests it is an evolving B-type star showing mixed traits of a subgiant or giant star. It is a PGa star – a sub-class of the higher temperature chemically peculiar stars known as mercury-manganese stars (HgMn stars). That is, it displays a rich spectra of singly-ionized phosphorus and gallium, in addition to ionized mercury and manganese. As such, Theta Hydri forms a typical example of this type. The absorption 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 a helium-weak star, having helium lines that are anomalously weak for its spectral type.[6] A weak and variable longitudinal magnetic field has been detected.[10]
There is a nearby companion star of class A0 IV[11] located at an angular separation of 0.1 arc seconds along a position 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 probable binary star system.[11]