21 Arietis

21 Arietis (abbreviated 21 Ari) is a binary star^[3] system in the northern constellation of Aries. 21 Arietis is the Flamsteed designation. It has a combined apparent visual magnitude is 5.57;^[2] the brighter member is magnitude 6.40 while the fainter star is magnitude 6.48.^[3] The distance to this star system, based upon an annual parallax shift of 19.58 mas,^[1] is 167 light-years (51 parsecs). The pair orbit each other with a period of 23.70 years and an eccentricity of 0.68.^[3]

21 Arietis

Observation data Epoch J2000      Equinox J2000
Constellation	Aries
Right ascension	02h 15m 42.77662s[1]
Declination	+25° 02′ 34.9627″[1]
Apparent magnitude (V)	5.57[2] (6.40/6.48)[3]
Characteristics
Spectral type	F6 V[4]
U−B color index	+0.00[5]
B−V color index	+0.50[5]
Astrometry
Radial velocity (Rv)	−44.3 km/s
Proper motion (μ)	RA: −89.72[1] mas/yr Dec.: −86.42[1] mas/yr
Parallax (π)	19.58±0.61 mas[1]
Distance	167 ± 5 ly (51 ± 2 pc)
Absolute magnitude (MV)	2.03[2]
Orbit[6]
Period (P)	23.70 ± 0.12 yr
Semi-major axis (a)	0.2353 ± 0.0011″
Eccentricity (e)	0.6816 ± 0.0037
Inclination (i)	104.52 ± 0.16°
Longitude of the node (Ω)	236.44 ± 0.22°
Periastron epoch (T)	1986.192 ± 0.012
Argument of periastron (ω) (secondary)	84.16 ± 0.16°
Details
21 Ari A
Mass	1.338 ± 0.032[7] M☉
Surface gravity (log g)	4.12[8] cgs
Temperature	6,299[8] K
Metallicity [Fe/H]	+0.02[8] dex
Rotational velocity (v sin i)	12.6[9] km/s
Age	2.2[2] Gyr
21 Ari B
Mass	1.374 ± 0.027[7] M☉
Other designations
BD+24 329, FK5 1059, HD 13872, HIP 10535, HR 657, SAO 75238, WDS J02157+2503.[5]
Database references
SIMBAD	data

The system was initially thought to be a triple system in 1981, since the orbit of the system predicted a mass greater than would be expected from its F6V spectral type.^[10] This was later rejected because the distance to the system was overestimated. However, while observing the spectrum of the system, it was found that a giant planet may be causing radial velocity variations. The purported planet would have a mass of 1.40 ± 0.36 M_J, an orbital period of 925 days and orbit the primary star.^[7]