21 Arietis

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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]
Spectral type F6 V[4]
U−B color index +0.00[5]
B−V color index +0.50[5]
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[1] mas
Distance167 ± 5 ly
(51 ± 2 pc)
Absolute magnitude (MV)2.03[2]
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)
84.16 ± 0.16°
21 Ari A
Mass1.338 ± 0.032[7] M
Surface gravity (log g)4.12[8] cgs
Temperature6,299[8] K
Metallicity [Fe/H]+0.02[8] dex
Rotational velocity (v sin i)12.6[9] km/s
Age2.2[2] Gyr
21 Ari B
Mass1.374 ± 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

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]

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 an mass of 1.40 ± 0.36 MJ, an orbital period of 925 days and orbit the primary star.[7]


  1. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357.
  2. ^ a b c d Holmberg, J.; Nordström, B.; Andersen, J. (July 2009), "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics", Astronomy and Astrophysics, 501 (3): 941–947, arXiv:0811.3982, Bibcode:2009A&A...501..941H, doi:10.1051/0004-6361/200811191.
  3. ^ a b c d Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x.
  4. ^ Harlan, E. A. (September 1969), "MK classifications for F- and G-type stars. I", Astronomical Journal, 74: 916–919, Bibcode:1969AJ.....74..916H, doi:10.1086/110881.
  5. ^ a b c "* 21 Ari". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-07-18.
  6. ^ Hartkopf, William I.; Mason, Brian D.; McAlister, Harold A. (January 1996), "Binary Star Orbits From Speckle Interferometry. VIII. Orbits of 37 Close Visual System", Astronomical Journal, 111: 370, Bibcode:1996AJ....111..370H, doi:10.1086/117790.
  7. ^ a b c Muterspaugh, Matthew W.; Lane, Benjamin F.; Kulkarni, S. R.; Konacki, Maciej; Burke, Bernard F.; Colavita, M. M.; Shao, M.; Hartkopf, William I.; Boss, Alan P.; Williamson, M. (2010). "The Phases Differential Astrometry Data Archive. V. Candidate Substellar Companions to Binary Systems". The Astronomical Journal. 140 (6): 1657. arXiv:1010.4048. Bibcode:2010AJ....140.1657M. doi:10.1088/0004-6256/140/6/1657.
  8. ^ a b c Balachandran, Suchitra (May 1, 1990), "Lithium depletion and rotation in main-sequence stars", Astrophysical Journal, Part 1, 354: 310–332, Bibcode:1990ApJ...354..310B, doi:10.1086/168691.
  9. ^ Schröder, C.; Reiners, A.; Schmitt, J. H. M. M. (January 2009), "Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo" (PDF), Astronomy and Astrophysics, 493 (3): 1099–1107, Bibcode:2009A&A...493.1099S, doi:10.1051/0004-6361:200810377.
  10. ^ Couteau, P.; Morel, P. J. (1982). "Is 21 Ari = Cou 79 a multiple system?". Astronomy and Astrophysics. 105: 323. Bibcode:1982A&A...105..323C.

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