HD 81799

HD 81799 (G Hydrae) is a suspected astrometric binary^[8] star system in the equatorial constellation of Hydra. It is visible to the naked eye with an apparent visual magnitude of 4.69.^[2] The distance to this system, as determined from an annual parallax shift of 19.9 mas,^[1] is 164 light years. It is moving further away from the Earth with a heliocentric radial velocity of 29 km/s.^[1] The system has a relatively high rate of proper motion, traversing the celestial sphere at the rate of 233±19 mas/yr along a position angle of 136°.^[9]

HD 81799

Observation data Epoch J2000      Equinox J2000
Constellation	Hydra
Right ascension	09h 27m 18.43258s[1]
Declination	−22° 20′ 37.4967″[1]
Apparent magnitude (V)	4.69[2]
Characteristics
Spectral type	K2+ IIIb[3]
B−V color index	1.154[4]
Astrometry
Radial velocity (Rv)	29.05±0.28[1] km/s
Proper motion (μ)	RA: +160.160[1] mas/yr Dec.: −173.131[1] mas/yr
Parallax (π)	19.8887 ± 0.3128 mas[1]
Distance	164 ± 3 ly (50.3 ± 0.8 pc)
Absolute magnitude (MV)	1.32[5]
Details
Radius	10.6[6] R☉
Luminosity	41.97[5] L☉
Surface gravity (log g)	2.43[2] cgs
Temperature	4,490[2] K
Metallicity [Fe/H]	−0.01[2] dex
Other designations
G Hya, BD−21° 2802, HD 81799, HIP 46371, HR 3749, SAO 177469, LTT 3479[7]
Database references
SIMBAD	data

The stellar classification of the visible component is K2+ IIIb,^[3] which matches an evolved K-type giant star. It is a red clump star, which indicates it is on the horizontal branch and is generating energy through helium fusion at its core.^[4] The interferometry-measured angular diameter of the primary, after correcting for limb darkening, is 1.96±0.03 mas,^[10] which, at its estimated distance, equates to a physical radius of about 10.6 times the radius of the Sun.^[6] It is radiating 42^[5] times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,490 K.^[2]

The system is a likely (99.4% chance) source of the X-ray emission coming from these coordinates.^[11]

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.
2. Soubiran, C.; Le Campion, J.-F.; Cayrel de Strobel, G.; Caillo, A. (June 2010), "The PASTEL catalogue of stellar parameters", Astronomy and Astrophysics, 515: A111, arXiv:1004.1069, Bibcode:2010A&A...515A.111S, doi:10.1051/0004-6361/201014247, S2CID 118362423.
3. Keenan, P.; McNeil, R. (October 1989), "The Perkins catalog of revised MK types for the cooler stars", Astrophysical Journal Supplement Series, 71: 245–266, Bibcode:1989ApJS...71..245K, doi:10.1086/191373
4. Alves, David R. (August 2000), "K-Band Calibration of the Red Clump Luminosity", The Astrophysical Journal, 539 (2): 732–741, arXiv:astro-ph/0003329, Bibcode:2000ApJ...539..732A, doi:10.1086/309278, S2CID 16673121.
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, S2CID 119257644.
6. Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, vol. 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R_*) is given by:

   ${\displaystyle {\begin{aligned}2\cdot R_{*}&={\frac {(10^{-3}\cdot 50.3\cdot 1.96)\ {\text{AU}}}{0.0046491\ {\text{AU}}/R_{\bigodot }}}\\&\approx 21.2\cdot R_{\bigodot }\end{aligned}}}$

7. "HD 81799". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-07-24.
8. Eggleton, P. P.; Tokovinin, A. A. (2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
9. Wroblewski, H.; Torres, C. (March 1998), "New proper motion determination of Luyten catalogue stars (LTT) with declination between -5° and -30° and right ascension between 0h and 13h 30m", Astronomy and Astrophysics Supplement, 128: 457–458, Bibcode:1998A&AS..128..457W, doi:10.1051/aas:1998157.
10. Richichi, A.; et al. (February 2005), "CHARM2: An updated Catalog of High Angular Resolution Measurements", Astronomy and Astrophysics, 431 (2): 773–777, Bibcode:2005A&A...431..773R, doi:10.1051/0004-6361:20042039
11. Haakonsen, Christian Bernt; Rutledge, Robert E. (September 2009), "XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources", The Astrophysical Journal Supplement, 184 (1): 138–151, arXiv:0910.3229, Bibcode:2009ApJS..184..138H, doi:10.1088/0067-0049/184/1/138, S2CID 119267456.