Chi2 Hydrae

Chi2 Hydrae, Latinised from χ2 Hydrae, is a binary star system in the equatorial constellation of Hydra. Based upon an annual parallax shift of 4.6 mas as seen from Earth, it is located roughly 710 light years from the Sun. It is visible to the naked eye with a combined apparent visual magnitude of 5.71.[2]

For other stars with this Bayer designation, see χ Hydrae
Chi2 Hydrae
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Hydra
Right ascension 11h 05m 57.57018s[1]
Declination −27° 17′ 16.2709″[1]
Apparent magnitude (V) 5.71[2] (5.85 + 7.57)[3]
Characteristics
Spectral type B8 III-IVe + B8.5 V[4]
U−B color index −0.26[2]
B−V color index −0.06[2]
Astrometry
Radial velocity (Rv)+30.6±0.9[5] km/s
Proper motion (μ) RA: +34.76[1] mas/yr
Dec.: −17.04[1] mas/yr
Parallax (π)4.6032 ± 0.0949[6] mas
Distance710 ± 10 ly
(217 ± 4 pc)
Orbit[7]
Period (P)2.2677 d
Eccentricity (e)0.00
Periastron epoch (T)2439925.545 JD
Argument of periastron (ω)
(secondary)		0.00°
Semi-amplitude (K1)
(primary)		123.3 km/s
Semi-amplitude (K2)
(secondary)		168.9 km/s
Details[8]
χ Lyr A
Mass3.605±0.078 M☉
Radius4.391±0.039 R☉
Luminosity344[9] L☉
Surface gravity (log g)3.65[10] cgs
Temperature11,750±190 K
Rotational velocity (v sin i)112±10[3] km/s
Age158[3] Myr
χ Lyr B
Mass2.632±0.049 M☉
Radius2.160±0.030 R☉
Luminosity66[9] L☉
Surface gravity (log g)4.23[3] cgs
Temperature11,100±230 K
Rotational velocity (v sin i)60±6[3] km/s
Other designations
χ Lyr, CD−26°8342, HD 96314, HIP 54255, HR 4317, SAO 179522[11]
Database references
SIMBADdata

This is a detached eclipsing binary star system with an orbital period of 2.27 days and an essentially circular orbit having a measured eccentricity of 0.00.[7] The eclipse of the primary by the secondary component reduces the visual magnitude of the system by 0.29, while the eclipse of the secondary diminishes the magnitude by 0.27.[4]

The primary, component A, is a magnitude 5.85 B-type star with a stellar classification of B8 III-IVe,[3] suggesting it may be part way along the path of evolving into a giant star from a subgiant. It has about 3.6 times the mass of the Sun and 4.4 times the Sun's radius,[8] although it may be tidally deformed since its radius is 86%[9] of the Roche radius.[12] With an estimated age of 158 million years, it has a projected rotational velocity of 112 km/s.[3]

Component B is a magnitude 7.57[3] B-type main sequence star with a class of B8.5 V.[4] It has 2.6 times the Sun's mass and 2.16 times the radius of the Sun. The star is filling 60% of its Roche radius.[9]

References
  1. 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. Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode:1978A&AS...34....1N.
  3. Torres, G.; et al. (February 2010), "Accurate masses and radii of normal stars: modern results and applications", The Astronomy and Astrophysics Review, 18 (1–2): 67–126, arXiv:0908.2624, Bibcode:2010A&ARv..18...67T, doi:10.1007/s00159-009-0025-1.
  4. Malkov, O. Yu.; et al. (February 2006), "A catalogue of eclipsing variables", Astronomy and Astrophysics, 446 (2): 785–789, Bibcode:2006A&A...446..785M, doi:10.1051/0004-6361:20053137.
  5. 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.
  6. 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.
  7. Pourbaix, D.; Tokovinin, A. A.; Batten, A. H.; Fekel, F. C.; Hartkopf, W. I.; et al. (2004), "SB9: The ninth catalogue of spectroscopic binary orbits", Astronomy & Astrophysics, 424 (2): 727–732, arXiv:astro-ph/0406573, Bibcode:2004A&A...424..727P, doi:10.1051/0004-6361:20041213.
  8. Brown, Timothy M. (January 2010), "Radii of Rapidly Rotating Stars, with Application to Transiting-Planet Hosts", The Astrophysical Journal, 709 (1): 535–545, arXiv:0912.1639, Bibcode:2010ApJ...709..535B, doi:10.1088/0004-637X/709/1/535.
  9. Eker, Z.; et al. (April 2015), "Main-Sequence Effective Temperatures from a Revised Mass-Luminosity Relation Based on Accurate Properties", The Astronomical Journal, 149 (4): 16, arXiv:1501.06585, Bibcode:2015AJ....149..131E, doi:10.1088/0004-6256/149/4/131, 131.
  10. Jordi, C.; Ribas, I.; Torra, J.; Gimenez, A. (October 1997), "Photometric versus empirical surface gravities of eclipsing binaries.", Astronomy and Astrophysics, 326: 1044–1054, Bibcode:1997A&A...326.1044J.
  11. "chi02 Hya". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-03-17.
  12. Pols, Onno R.; et al. (August 1997), "Further critical tests of stellar evolution by means of double-lined eclipsing binaries", Monthly Notices of the Royal Astronomical Society, 289 (4): 869–881, Bibcode:1997MNRAS.289..869P, doi:10.1093/mnras/289.4.869.
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