Theta Hydri

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]

Theta Hydri

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]