HD 81799
Star and suspected binary system in the constellation Hydra
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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]
 | |
| 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]
