# Theta Hydrae

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Binary star system in the constellation Hydra

Theta Hydrae Location of θ Hydrae (circled in red) 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 mas[1] Distance 115 ± 1 ly (35.2 ± 0.5 pc) Absolute magnitude (MV) +0.92[5] Details θ Hya A Mass 2.52[3] M☉ Luminosity 52[5] L☉ Surface gravity (log g) 3.80±0.08[6] cgs Temperature 10,099±145[6] K Metallicity [Fe/H] −0.42±0.09[6] dex Rotational velocity (v sin i) 95[7] km/s θ Hya B Mass 0.68[8] or 1.21[3] M☉ Temperature 30,700[3] K Other designations θ Hya, 22 Hydrae, BD+02°2167, FK5 347, HD 79469, HIP 45336, HR 3665, SAO 117527[9] Database references SIMBAD data

**Theta Hydrae**, [Latinized](/source/Latinisation_of_names) from **θ Hydrae**, is a [binary star](/source/Binary_star)[8] system in the [constellation](/source/Constellation) [Hydra](/source/Hydra_(constellation)). It is visible to the naked eye with an [apparent visual magnitude](/source/Apparent_visual_magnitude) of 3.9.[2] The star system has a high [proper motion](/source/Proper_motion)[8] with an annual [parallax shift](/source/Stellar_parallax) of 28.4 [mas](/source/Milliarcsecond),[1] indicating a distance of about 115 [light years](/source/Light_years). Theta Hydrae forms a [double](/source/Double_star) with a magnitude 9.9 star located at an [angular separation](/source/Angular_separation) of 29 [arcseconds](/source/Arcseconds).[10]

The primary component of this system is a [B-type main sequence star](/source/B-type_main_sequence_star) with a [stellar classification](/source/Stellar_classification) of B9.5 V.[3] It is a candidate [Lambda Boötis star](/source/Lambda_Bo%C3%B6tis_star), indicating it displays an underabundance of [iron peak](/source/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](/source/Ap_and_Bp_stars).[12]

An orbiting [white dwarf](/source/White_dwarf) companion was discovered in 1998 from its [X-ray emission](/source/X-ray_astronomy). This [degenerate star](/source/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](/source/Solar_mass), 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. ^ [***a***](#cite_ref-dr2_1-0) [***b***](#cite_ref-dr2_1-1) [***c***](#cite_ref-dr2_1-2) [***d***](#cite_ref-dr2_1-3) [***e***](#cite_ref-dr2_1-4) [***f***](#cite_ref-dr2_1-5) [Brown, A. G. A.](/source/Anthony_Brown_(scientist)); et al. ([Gaia collaboration](/source/Data_Processing_and_Analysis_Consortium)) (August 2018). ["*Gaia* Data Release 2: Summary of the contents and survey properties"](https://doi.org/10.1051%2F0004-6361%2F201833051). *[Astronomy & Astrophysics](/source/Astronomy_%26_Astrophysics)*. **616**. A1. [arXiv](/source/ArXiv_(identifier)):[1804.09365](https://arxiv.org/abs/1804.09365). [Bibcode](/source/Bibcode_(identifier)):[2018A&A...616A...1G](https://ui.adsabs.harvard.edu/abs/2018A&A...616A...1G). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201833051](https://doi.org/10.1051%2F0004-6361%2F201833051). [Gaia DR2 record for this source](https://vizier.u-strasbg.fr/viz-bin/VizieR-S?Gaia%20DR2%203845263368043086080) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-Cousins1984_2-0) [***b***](#cite_ref-Cousins1984_2-1) [***c***](#cite_ref-Cousins1984_2-2) [***d***](#cite_ref-Cousins1984_2-3) Cousins, A. W. J. (1984), "Standardization of Broadband Photometry of Equatorial Standards", *South African Astronomical Observatory Circulars*, **8**: 59, [Bibcode](/source/Bibcode_(identifier)):[1984SAAOC...8...59C](https://ui.adsabs.harvard.edu/abs/1984SAAOC...8...59C).

1. ^ [***a***](#cite_ref-Holberg2013_3-0) [***b***](#cite_ref-Holberg2013_3-1) [***c***](#cite_ref-Holberg2013_3-2) [***d***](#cite_ref-Holberg2013_3-3) [***e***](#cite_ref-Holberg2013_3-4) [***f***](#cite_ref-Holberg2013_3-5) 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](/source/ArXiv_(identifier)):[1307.8047](https://arxiv.org/abs/1307.8047), [Bibcode](/source/Bibcode_(identifier)):[2013MNRAS.435.2077H](https://ui.adsabs.harvard.edu/abs/2013MNRAS.435.2077H), [doi](/source/Doi_(identifier)):[10.1093/mnras/stt1433](https://doi.org/10.1093%2Fmnras%2Fstt1433).

1. **[^](#cite_ref-Gontcharov2006_4-0)** 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](/source/ArXiv_(identifier)):[1606.08053](https://arxiv.org/abs/1606.08053), [Bibcode](/source/Bibcode_(identifier)):[2006AstL...32..759G](https://ui.adsabs.harvard.edu/abs/2006AstL...32..759G), [doi](/source/Doi_(identifier)):[10.1134/S1063773706110065](https://doi.org/10.1134%2FS1063773706110065), [S2CID](/source/S2CID_(identifier)) [119231169](https://api.semanticscholar.org/CorpusID:119231169).

1. ^ [***a***](#cite_ref-Anderson2012_5-0) [***b***](#cite_ref-Anderson2012_5-1) Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", *Astronomy Letters*, **38** (5): 331, [arXiv](/source/ArXiv_(identifier)):[1108.4971](https://arxiv.org/abs/1108.4971), [Bibcode](/source/Bibcode_(identifier)):[2012AstL...38..331A](https://ui.adsabs.harvard.edu/abs/2012AstL...38..331A), [doi](/source/Doi_(identifier)):[10.1134/S1063773712050015](https://doi.org/10.1134%2FS1063773712050015), [S2CID](/source/S2CID_(identifier)) [119257644](https://api.semanticscholar.org/CorpusID:119257644).

1. ^ [***a***](#cite_ref-Wu2011_6-0) [***b***](#cite_ref-Wu2011_6-1) [***c***](#cite_ref-Wu2011_6-2) Wu, Yue; et al. (January 2011), "Coudé-feed stellar spectral library – atmospheric parameters", *Astronomy and Astrophysics*, **525**: A71, [arXiv](/source/ArXiv_(identifier)):[1009.1491](https://arxiv.org/abs/1009.1491), [Bibcode](/source/Bibcode_(identifier)):[2011A&A...525A..71W](https://ui.adsabs.harvard.edu/abs/2011A&A...525A..71W), [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201015014](https://doi.org/10.1051%2F0004-6361%2F201015014), [S2CID](/source/S2CID_(identifier)) [53480665](https://api.semanticscholar.org/CorpusID:53480665).

1. **[^](#cite_ref-Zorec2012_7-0)** Zorec, J.; Royer, F. (January 2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", *Astronomy & Astrophysics*, **537**: A120, [arXiv](/source/ArXiv_(identifier)):[1201.2052](https://arxiv.org/abs/1201.2052), [Bibcode](/source/Bibcode_(identifier)):[2012A&A...537A.120Z](https://ui.adsabs.harvard.edu/abs/2012A&A...537A.120Z), [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201117691](https://doi.org/10.1051%2F0004-6361%2F201117691), [S2CID](/source/S2CID_(identifier)) [55586789](https://api.semanticscholar.org/CorpusID:55586789).

1. ^ [***a***](#cite_ref-Burleigh1999_8-0) [***b***](#cite_ref-Burleigh1999_8-1) [***c***](#cite_ref-Burleigh1999_8-2) [***d***](#cite_ref-Burleigh1999_8-3) [***e***](#cite_ref-Burleigh1999_8-4) 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](/source/ArXiv_(identifier)):[astro-ph/9810113](https://arxiv.org/abs/astro-ph/9810113), [Bibcode](/source/Bibcode_(identifier)):[1999A&A...341..795B](https://ui.adsabs.harvard.edu/abs/1999A&A...341..795B).

1. **[^](#cite_ref-SIMBAD_9-0)** ["tet Hya"](http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=tet+Hya). *[SIMBAD](/source/SIMBAD)*. [Centre de données astronomiques de Strasbourg](/source/Centre_de_donn%C3%A9es_astronomiques_de_Strasbourg). Retrieved 2017-01-04.{{[cite web](https://en.wikipedia.org/wiki/Template:Cite_web)}}: CS1 maint: postscript ([link](https://en.wikipedia.org/wiki/Category:CS1_maint:_postscript))

1. **[^](#cite_ref-Privett2013_10-0)** Privett, Grant; Jones, Kevin (2013), [*The Constellation Observing Atlas*](https://books.google.com/books?id=uN69BAAAQBAJ&pg=PA104), The Patrick Moore Practical Astronomy Series, Springer Science & Business Media, p. 104, [Bibcode](/source/Bibcode_(identifier)):[2013coa..book.....P](https://ui.adsabs.harvard.edu/abs/2013coa..book.....P), [ISBN](/source/ISBN_(identifier)) [978-1461476481](https://en.wikipedia.org/wiki/Special:BookSources/978-1461476481).

1. **[^](#cite_ref-King1994_11-0)** King, J. R. (July 1994), ["Accretion from Circumstellar Discs and the Lambda-Bootis Phenomenon"](http://tigerprints.clemson.edu/cgi/viewcontent.cgi?article=1236&context=physastro_pubs), *[Monthly Notices of the Royal Astronomical Society](/source/Monthly_Notices_of_the_Royal_Astronomical_Society)*, **269** (1): 209–217, [Bibcode](/source/Bibcode_(identifier)):[1994MNRAS.269..209K](https://ui.adsabs.harvard.edu/abs/1994MNRAS.269..209K), [doi](/source/Doi_(identifier)):[10.1093/mnras/269.1.209](https://doi.org/10.1093%2Fmnras%2F269.1.209).

1. **[^](#cite_ref-Baschek1969_12-0)** Baschek, Bodo; Searle, Leonard (February 1969), "The Chemical Composition of the Lambda Bootis Stars", *Astrophysical Journal*, **155**: 537, [Bibcode](/source/Bibcode_(identifier)):[1969ApJ...155..537B](https://ui.adsabs.harvard.edu/abs/1969ApJ...155..537B), [doi](/source/Doi_(identifier)):[10.1086/149890](https://doi.org/10.1086%2F149890).

1. **[^](#cite_ref-Vennes2008_13-0)** 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](/source/ArXiv_(identifier)):[0806.4742](https://arxiv.org/abs/0806.4742), [Bibcode](/source/Bibcode_(identifier)):[2008MNRAS.389.1367V](https://ui.adsabs.harvard.edu/abs/2008MNRAS.389.1367V), [doi](/source/Doi_(identifier)):[10.1111/j.1365-2966.2008.13652.x](https://doi.org/10.1111%2Fj.1365-2966.2008.13652.x), [S2CID](/source/S2CID_(identifier)) [15349194](https://api.semanticscholar.org/CorpusID:15349194).

## External links

- Kaler, James B. (May 8, 2015), ["Theta Hydrae"](http://stars.astro.illinois.edu/sow/thetahya.html), *Stars*, University of Illinois, retrieved 2016-01-04.

v t e Constellation of Hydra List of stars in Hydra Hydra in Chinese astronomy Stars Bayer α (Alphard) β γ (Naga) δ ε (Ashlesha) ζ η θ ι (Ukdah) κ λ μ ν ξ ο π ρ σ (Minchir) τ1 τ2 υ1 (Zhang) υ2 φ1 φ2 φ3 χ1 χ2 ψ ω b1 b3 C F Flamsteed 1 2 3 6 (a) 9 10 12 (D) 14 15 17 19 20 21 23 24 25 26 27 (P) 28 29 33 (A) 34 37 44 47 48 50 51 (k) 52 (l) 54 (m) 55 56 57 58 (Solitaire) 59 60 10 Crt 17 Crt 20 Crt 2 Sex Variable R U V W TW EX HS KU LQ V361 V419 V421 V478 V484 HR 3378 3538 3749 (G) 3750 3858 (I) 3862 3919 3923 (Felis) 4162 4328 4339 (β Ant) 4445 5265 HD 72659 74156 82943 86226 86264 90156 (γ Ant) Other 2MASS 1114−2618 ESO 439-26 Gliese 328 Gliese 357 Gliese 433 Gliese 453 GJ 3634 HAT-P-30 HE 1256−2738 HE 1327−2326 LHS 2065 LHS 3003 LTT 3780 SDSS J090745.0+024507 WASP-25 WASP-36 WASP-84 WASP-166 (Filetdor) WISE 0855−0714 WISEA J1141−3326 Exoplanets Gliese 357 d HD 72659 b HD 74156 b c HD 82943 b c HD 86226 b HD 86264 b TW Hydrae b (unconfirmed) WASP-193b Star clusters Messier 48 Messier 68 NGC 5694 Price-Whelan 1 Nebulae Abell 33 Abell 35 NGC 3242 Southern Owl Nebula Galaxies Messier 83 NGC 2617 2642 2697 2708 2758 2835 2848 2865 2890 2935 2936 2937 2960 2962 2986 2992 3030 3052 3054 3081 3109 3124 3200 3285 3285B 3290 3305 3307 3308 3309 3311 3312 3313 3314 3315 3316 3336 3369 3383 3393 3402 3463 3464 3585 3621 3673 3717 3904 3923 3936 4980 4993 5042 5061 5078 5085 5101 5135 5260 5264 5495 5626 Other 3C 196.1 AM 1316−241 Arp 7 CTQ 327 ESO 510-G13 GAMA202627 GLEAM J0917−0012 Hydra A IC 535 MRC 1138−262 Peekaboo Galaxy PKS 1151−348 RX J0911.4+0551 SDSS J0841+0101 TN J0924−2201 Tololo 1247−232 ZwCl 1693 BCG Galaxy clusters Abell 754 Abell 3411 Abell 3412 Bullet Group Hickson 40 Astronomical events GW170817 Category

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Adapted from the Wikipedia article [Theta Hydrae](https://en.wikipedia.org/wiki/Theta_Hydrae) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Theta_Hydrae?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.