Theta Hydrae

Theta Hydrae, Latinized from θ Hydrae, is a binary star[8] system in the constellation Hydra. It is visible to the naked eye with an apparent visual magnitude of 3.9.[2] The star system has a high proper motion[8] with an annual parallax shift of 28.4 mas,[1] indicating a distance of about 115 light years. Theta Hydrae forms a double with a magnitude 9.9 star located at an angular separation of 29 arcseconds.[10]

Theta Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 09h 14m 21.866s[1]
Declination +02° 18′ 51.64″[1]
Apparent magnitude (V) 3.888[2]
Characteristics
Spectral type B9.5 V + DA 1.6[3]
U−B color index −0.118[2]
B−V color index −0.065[2]
Astrometry
Radial velocity (Rv)−10.7±0.3[4] km/s
Proper motion (μ) RA: +128.152[1] mas/yr
Dec.: −327.709[1] mas/yr
Parallax (π)28.4019 ± 0.3682[1] mas
Distance115 ± 1 ly
(35.2 ± 0.5 pc)
Absolute magnitude (MV)+0.92[5]
Details
θ Hya A
Mass2.52[3] M☉
Luminosity52[5] L☉
Surface gravity (log g)3.80±0.08[6] cgs
Temperature10,099±145[6] K
Metallicity [Fe/H]−0.42±0.09[6] dex
Rotational velocity (v sin i)95[7] km/s
θ Hya B
Mass0.68[8] or 1.21[3] M☉
Temperature30,700[3] K
Other designations
θ Hya, 22 Hydrae, BD+02°2167, FK5 347, HD 79469, HIP 45336, HR 3665, SAO 117527[9]
Database references
SIMBADdata

The primary component of this system is a B-type main sequence star with a stellar classification of B9.5 V.[3] It is a candidate Lambda Boötis star, indicating it displays an underabundance of iron peak elements.[11] However, it is also underabundant in oxygen, a characteristic not shared by other Lambda Boötis stars. Instead, it may be a peculiar B star.[12]

An orbiting white dwarf companion was discovered in 1998 from its X-ray emission. This degenerate star must have evolved from a progenitor that was once more massive than the current primary.[8] Burleigh and Barstow (1999) gave a mass estimate of 0.68[8] times the mass of the Sun, whereas Holberg et al. (2013) put it as high as 1.21[3] times the Sun's mass. The latter would put it beyond the theoretical upper limit for white dwarf remnants of typical single stars that did not undergo a merger or mass loss.[13]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. Cousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", South African Astronomical Observatory Circulars, 8: 59, Bibcode:1984SAAOC...8...59C.
  3. Holberg, J. B.; et al. (November 2013), "Where are all the Sirius-like binary systems?", Monthly Notices of the Royal Astronomical Society, 435 (3): 2077–2091, arXiv:1307.8047, Bibcode:2013MNRAS.435.2077H, doi:10.1093/mnras/stt1433.
  4. Gontcharov, G. A. (November 2006), "Pulkovo Compilation of Radial Velocities for 35,495 Hipparcos stars in a common system", Astronomy Letters, 32 (11): 759–771, arXiv:1606.08053, Bibcode:2006AstL...32..759G, doi:10.1134/S1063773706110065.
  5. 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. Wu, Yue; et al. (January 2011), "Coudé-feed stellar spectral library – atmospheric parameters", Astronomy and Astrophysics, 525: A71, arXiv:1009.1491, Bibcode:2011A&A...525A..71W, doi:10.1051/0004-6361/201015014.
  7. Zorec, J.; Royer, F. (January 2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", Astronomy & Astrophysics, 537: A120, arXiv:1201.2052, Bibcode:2012A&A...537A.120Z, doi:10.1051/0004-6361/201117691.
  8. Burleigh, M. R.; Barstow, M. A. (January 1999), "Theta Hya: spectroscopic identification of a second B star+white dwarf binary", Astronomy and Astrophysics, 341: 795–798, arXiv:astro-ph/9810113, Bibcode:1999A&A...341..795B.
  9. "tet Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-01-04.
  10. Privett, Grant; Jones, Kevin (2013), The Constellation Observing Atlas, The Patrick Moore Practical Astronomy Series, Springer Science & Business Media, p. 104, ISBN 1461476488.
  11. King, J. R. (July 1994), "Accretion from Circumstellar Discs and the Lambda-Bootis Phenomenon", Monthly Notices of the Royal Astronomical Society, 269 (1): 209–217, Bibcode:1994MNRAS.269..209K, doi:10.1093/mnras/269.1.209.
  12. Baschek, Bodo; Searle, Leonard (February 1969), "The Chemical Composition of the Lambda Bootis Stars", Astrophysical Journal, 155: 537, Bibcode:1969ApJ...155..537B, doi:10.1086/149890.
  13. Vennes, S.; Kawka, A. (September 2008), "On the empirical evidence for the existence of ultramassive white dwarfs", Monthly Notices of the Royal Astronomical Society, 389 (3): 1367–1374, arXiv:0806.4742, Bibcode:2008MNRAS.389.1367V, doi:10.1111/j.1365-2966.2008.13652.x.
  • Kaler, James B. (May 8, 2015), "Theta Hydrae", Stars, University of Illinois, retrieved 2016-01-04.
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