Theta Hydri
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) |
|
|---|---|
| Constellation | Hydrus |
| Right ascension | 03h 02m 15.44844s[1] |
| Declination | −71° 54′ 08.8369″[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: +27.19[1] mas/yr Dec.: +16.82[1] mas/yr |
| Parallax (π) | 6.34 ± 0.20[1] mas |
| Distance | 510 ± 20 ly (158 ± 5 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 |
| Other designations | |
| Database references | |
| SIMBAD | data |
Theta Hydri, Latinized from θ Hydri, is the Bayer designation for 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 6.34 mas as seen from Earth,[1] is located roughly 510 light years from the Sun. 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]
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 higher temperature variety of the class of 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.[9] 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.[9]
There is a nearby companion star of class A0 IV[10] 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,[11] although Eggleton and Tokovinin (2008) listed the pair as a probable binary star system.[10]
References
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