66 Eridani

66 Eridani is a binary star in the constellation of Eridanus. The combined apparent magnitude of the system is 5.12 on average.^[2] Parallax measurements by Hipparcos put the system at some 309 light-years (95 parsecs) away.^[1] It lies half a degree northwest of beta Eridani.^[8]

66 Eridani

Location of 66 Eridani (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Eridanus
Right ascension	05h 06m 45.65314s[1]
Declination	−04° 39′ 18.5939″[1]
Apparent magnitude (V)	5.12[2]
Characteristics
Spectral type	B9V[3]
U−B color index	-0.16[4]
B−V color index	-0.06[4]
Variable type	α2 CVn?[5]
Astrometry
Radial velocity (Rv)	32.28±0.10[6] km/s
Proper motion (μ)	RA: 10.70[1] mas/yr Dec.: −0.78[1] mas/yr
Parallax (π)	10.56±0.34 mas[1]
Distance	309 ± 10 ly (95 ± 3 pc)
Absolute magnitude (MV)	0.37[7]
Orbit[6]
Period (P)	5.5226013 ± 0.0000020 d
Eccentricity (e)	0.0844±0.0013
Periastron epoch (T)	2441356.499±0.017
Argument of periastron (ω) (secondary)	160.9±1.1°
Semi-amplitude (K1) (primary)	102.83±0.20 km/s
Semi-amplitude (K2) (secondary)	100.35±0.19 km/s
Details[6]
66 Eri A
Mass	2.629±0.032 M☉
Radius	1.948±0.063 R☉
Luminosity	51.3±3.3 L☉
Surface gravity (log g)	4.28±0.03 cgs
Temperature	11,077 K
Rotational velocity (v sin i)	17.1±0.2 km/s
Age	30 Myr
66 Eri B
Mass	2.566±0.032 M☉
Radius	1.919±0.061 R☉
Luminosity	46.9±3.0 L☉
Surface gravity (log g)	4.28±0.03 cgs
Temperature	10,914 K
Rotational velocity (v sin i)	16.9±0.2 km/s
Other designations
EN Eri, BD−04°1044, HD 32964, HIP 23794, HR 1657, SAO 131777[8]
Database references
SIMBAD	data

This is a spectroscopic binary: the two stars cannot be individually resolved, but periodic Doppler shifts in its spectrum mean there must be orbital motion. The two stars orbit each other every 5.5 days.^[6] Their orbit is fairly circular, with an eccentricity of 0.0844.^[6]

The combined spectrum of 66 Eridani matches that of a B-type main-sequence star,^[3] and the two stars have similar masses.^[6] The spectrum also shows excess of mercury and manganese, as it is a type of chemically peculiar star called a mercury-manganese star.^[6] 66 Eridani is an Alpha² Canum Venaticorum variable. For this reason, it has been given the designation EN Eridani.