18 Monocerotis

18 Monocerotis is a binary star system located about halfway from Orion's Belt to Procyon,^[6] in the equatorial constellation of Monoceros.^[11] It is visible to the naked eye as a faint, orange-hued star with an apparent visual magnitude of 4.47,^[2] and is positioned around 370^[1] light years away from the Sun based on parallax. The system is receding from the Earth with a heliocentric radial velocity of +11 km/s.^[4]

18 Monocerotis

Location of 18 Monocerotis (circled red)
Observation data Epoch J2000      Equinox J2000
Constellation	Monoceros
Right ascension	06h 47m 51.64752s[1]
Declination	+02° 24′ 43.7737″[1]
Apparent magnitude (V)	4.47[2]
Characteristics
Spectral type	K0+IIIaBa0.2[3]
U−B color index	+1.04[2]
B−V color index	+1.11[2]
Astrometry
Radial velocity (Rv)	+11.29[4] km/s
Proper motion (μ)	RA: -16.898[1] mas/yr Dec.: -16.229[1] mas/yr
Parallax (π)	8.0947±0.2385 mas[1]
Distance	400 ± 10 ly (124 ± 4 pc)
Absolute magnitude (MV)	-0.78[5]
Orbit[6][7]
Primary	18 Monocerotis A
Companion	18 Monocerotis B
Period (P)	1760.9±1.9 d
Semi-major axis (a)	6.63 mas
Eccentricity (e)	0.40±0.04
Inclination (i)	96.25°
Longitude of the node (Ω)	47.63°
Periastron epoch (T)	2441942.5
Argument of periastron (ω) (secondary)	172±6°
Details
18 Mon A
Radius	26.95+1.56 −0.81[1] R☉
Luminosity	311.2±10.5[1] L☉
Surface gravity (log g)	2.59[8] cgs
Temperature	4,750[9] K
Metallicity [Fe/H]	-0.03[5] dex
Rotational velocity (v sin i)	< 2.0[10] km/s
Other designations
18 Mon, BD+02°1397, FK5 258, GC 8892, HD 49293, HIP 32578, HR 2506, SAO 114428[11]
Database references
SIMBAD	data

It is reported as a single-lined spectroscopic binary with an orbital period of 1,760.9 days (4.8 years) and an eccentricity of 0.4.^[6] The visible component is an aging K-type giant star with a stellar classification of K0+IIIaBa0.2,^[3] showing a slight overabundance of barium. The spectrum displays strong violet lines of CN.^[6] With the supply of hydrogen at its core exhausted, this star has expanded to 27^[1] times the radius of the Sun. It is radiating 311^[1] times the luminosity of the Sun from its swollen photosphere at an effective temperature of 4,750 K.^[9]