27 Hydrae

27 Hydrae is a triple star system^[6] system in the equatorial constellation of Hydra,^[10] located 222 light years away from the Sun.^[1] It is visible to the naked eye as a faint, orange-hued star with a combined apparent visual magnitude of 4.82.^[2] The system is moving further from the Earth with a heliocentric radial velocity of +25.6 km/s.^[7]

27 Hydrae

Observation data Epoch J2000      Equinox J2000
Constellation	Hydra
Right ascension	09h 20m 29.01857s[1]
Declination	−09° 33′ 20.5054″[1]
Apparent magnitude (V)	4.818[2] (4.91 + 7.03 + 10.99)[3]
Characteristics
Evolutionary stage	red clump[4]
Spectral type	K0III[5] + F4V + K2V[6]
Astrometry
Radial velocity (Rv)	+25.60±0.13[7] km/s
Proper motion (μ)	RA: −12.48[1] mas/yr Dec.: −27.37[1] mas/yr
Parallax (π)	14.66±0.31 mas[1]
Distance	222 ± 5 ly (68 ± 1 pc)
Details
A
Mass	2.17[2] M☉
Radius	11[7] R☉
Luminosity	57.5[2] L☉
Surface gravity (log g)	2.9[7] cgs
Temperature	4,965±26[2] K
Metallicity [Fe/H]	−0.07[7] dex
Rotational velocity (v sin i)	3.3[7] km/s
Age	1.91[2] Gyr
B
Radius	1.82[8] R☉
Luminosity	5.885[8] L☉
Temperature	6,664[8] K
C
Radius	0.72[9] R☉
Luminosity	0.227[9] L☉
Temperature	4,685[9] K
Other designations
CCDM J09204-0934, WDS J09204-0934[10]
A: 27 Hya, BD−08°2643, HD 80586, HIP 45811, HR 3709, SAO 136768
B: BD−09°2801, HD 80550, HIP 45802, SAO 136767
C: TYC 5463-1518-1
Database references
SIMBAD	data
	B
	C

The magnitude 4.91^[6] primary, component A, is an aging giant star with a stellar classification of K0 III.^[5] It is a red clump giant,^[4] which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. The star is 1.9^[2] billion years old with 2.17^[2] times the mass of the Sun. It has swelled to 11^[7] times the Sun's radius and is radiating 57.5^[2] times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,965 K.^[2] The star is suspected to host a low-mass companion.^[11]

The stellar companions to this star, designated components B and C, lie at an angular separation of 229.10″ from the primary, and form a binary pair with a separation of 9.20″ as of 2015.^[3] The brighter member of the pair, component B, is a seventh magnitude F-type main-sequence star with a class of F4 V, while its companion is an eleventh magnitude K-type main-sequence star with a class of K2 V.^[6]

Substellar companion

The Okayama Planet Search team published a paper in late 2008 reporting investigations into radial velocity variations observed for a set of evolved stars, showing hints of a substellar companion orbiting the primary member of the wide binary system 27 Hydrae.^[11] Its orbital period is estimated at 9.3 years, but no planet has been confirmed yet.

The 27 Hydrae planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b (unconfirmed)	≥10 MJ	≈5.9	3,400	—	—	—