Theta Hydri

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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]
Spectral type B8 III/IV[3]
U−B color index −0.51[2]
B−V color index −0.14[2]
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
Distance510 ± 20 ly
(158 ± 5 pc)
Absolute magnitude (MV)−0.48[5]
Luminosity287[5] L
Surface gravity (log g)3.76[6] cgs
Temperature13,350[6] K
Other designations
θ Hyi, CPD−72° 219, FK5 113, HD 19400, HIP 14131, HR 939, SAO 255945, WDS J03023-7154AB[7]
Database references

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]


  1. ^ a b c d e f van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357.
  2. ^ a b c d Johnson, H. L.; Mitchell, R. I.; Iriarte, B.; Wisniewski, W. Z. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99), Bibcode:1966CoLPL...4...99J.
  3. ^ a b Houk, N.; Cowley, A. P. (1975), Michigan catalogue of two-dimensional spectral types for the HD stars, 1, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan,
  4. ^ a b de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, A61.
  5. ^ a b Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  6. ^ a b c Alonso, M. S.; et al. (April 2003), "Elemental abundance studies of CP stars. The helium-weak stars HD 19400, HD 34797 and HD 35456*", Astronomy and Astrophysics, 402: 331−334, Bibcode:2003A&A...402..331A, doi:10.1051/0004-6361:20030222.
  7. ^ "tet Hyi". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-10-13.
  8. ^ Gontcharov, G. A. (November 2012), "Spatial distribution and kinematics of OB stars", Astronomy Letters, 38 (11): 694−706, arXiv:1606.09028, Bibcode:2012AstL...38..694G, doi:10.1134/S1063773712110035.
  9. ^ a b Hubrig, S.; et al. (August 2014), "Abundance analysis, spectral variability, and search for the presence of a magnetic field in the typical PGa star HD 19400", Monthly Notices of the Royal Astronomical Society, 442 (4): 3604−3615, arXiv:1406.1927, Bibcode:2014MNRAS.442.3604H, doi:10.1093/mnras/stu1122.
  10. ^ a b Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x.
  11. ^ Schöller, M.; et al. (November 2010), "Multiplicity of late-type B stars with HgMn peculiarity", Astronomy and Astrophysics, 522: 12, arXiv:1010.3643, Bibcode:2010A&A...522A..85S, doi:10.1051/0004-6361/201014246, A85