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.^[3] The distance to this system, as determined from an annual parallax shift of 19.9 mas,^[2] is 164 light years. It is moving further away from the Earth with a heliocentric radial velocity of 29 km/s.^[2] 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[1]
Right ascension	09h 27m 18.43258s[2]
Declination	−22° 20′ 37.4967″[2]
Apparent magnitude (V)	4.69[3]
Characteristics
Evolutionary stage	red clump[4]
Spectral type	K2+ IIIb[5]
B−V color index	1.154[4]
Astrometry
Radial velocity (Rv)	29.05±0.28[2] km/s
Proper motion (μ)	RA: +160.160[2] mas/yr Dec.: −173.131[2] mas/yr
Parallax (π)	19.8887±0.3128 mas[2]
Distance	164 ± 3 ly (50.3 ± 0.8 pc)
Absolute magnitude (MV)	1.32[1]
Details
Radius	10.6[6] R☉
Luminosity	41.97[1] L☉
Surface gravity (log g)	2.43[3] cgs
Temperature	4,490[3] K
Metallicity [Fe/H]	−0.01[3] 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,^[5] 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^[1] times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,490 K.^[3]

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