4 Equulei

4 Equulei is a binary star system in the constellation of Equuleus, located about 3° to the east of Alpha Equulei.[9] With an apparent visual magnitude of 5.94,[2] it is just barely visible to the naked eye under good seeing conditions. The system shows an annual parallax shift of 20.44 mas as measured from Earth's orbit, which yields a distance estimate of about 160 light years. It is moving closer to the Sun with a radial velocity of −13 km/s.[5]

4 Equulei
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Equuleus
Right ascension 21h 05m 26.71378s[1]
Declination +05° 57′ 29.5655″[1]
Apparent magnitude (V) 5.94[2]
Characteristics
Spectral type F8 V[3] + ?
U−B color index 0.20[4]
B−V color index 0.538[2]
Astrometry
Radial velocity (Rv)−12.7±0.3[5] km/s
Proper motion (μ) RA: −128.69[1] mas/yr
Dec.: −113.60[1] mas/yr
Parallax (π)20.44 ± 1.68[1] mas
Distance160 ± 10 ly
(49 ± 4 pc)
Absolute magnitude (MV)2.52[2]
Orbit[2]
Period (P)1975.76±0.94 d
Eccentricity (e)0.3937±0.0047
Periastron epoch (T)51004.2 ± 4.2 HJD
Argument of periastron (ω)
(secondary)		14.22±0.83°
Semi-amplitude (K1)
(primary)		10.585±0.061 km/s
Details[6]
Mass1.39+0.09
−0.03 M☉
Radiusapprox 1.2[7] R☉
Luminosity4.98[5] L☉
Surface gravity (log g)43.80±0.06 cgs
Temperature6213±63 K
Metallicity [Fe/H]−0.18±0.05 dex
Rotational velocity (v sin i)6.2±1.0[2] km/s
Age3.07+0.35
−0.44 Gyr
Other designations
4 Equ, BD+05° 4697, HD 200790, HIP 104101, HR 8077, SAO 126535, WDS J21054+0557A[8]
Database references
SIMBADdata

This is a single-lined spectroscopic binary with an orbital period of 5.4093 ± 0.0026 years (1,975.76 ± 0.94 d) and an eccentricity of 0.39. The value of a sin i[10] for the primary is (264.4±1.6)×106 km, providing a lower bound for the semimajor axis.[2] Griffin (2011) noted that the mass of the secondary component is likely to be at least equal to that of the Sun, which, for an ordinary main sequence star, should make it visible in the spectrum. The lack of significant ultraviolet emission appears to rule out a white dwarf companion. Instead, the secondary may consist of a binary pair of low mass dwarfs.[9]

The visible component is an F-type main-sequence star with a stellar classification of F8 V.[3] The spectrum of the star is unusual for its near absence of lithium.[11] With an estimated age of about three billion years, it has 1.4[6] times the mass of the Sun and is spinning with a projected rotational velocity of 6 km/s.[2] It is radiating five[5] times the Sun's luminosity from its photosphere at an effective temperature of around 6,213 K.[6] 4 Equulei is not known to harbor a planet.[11] It has a magnitude 12.4 companion star located, as of 2012, at an angular separation of 30.70 arcseconds along a position angle of 301°.[12]

References
  1. van Leeuwen, F. (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. Willmarth, Daryl W.; et al. (August 2016), "Spectroscopic Orbits for 15 Late-type Stars" (PDF), The Astronomical Journal, 152 (2): 13, Bibcode:2016AJ....152...46W, doi:10.3847/0004-6256/152/2/46, 46.
  3. Takeda, Yoichi; et al. (February 2005), "High-Dispersion Spectra Collection of Nearby F--K Stars at Okayama Astrophysical Observatory: A Basis for Spectroscopic Abundance Standards", Publications of the Astronomical Society of Japan, 57 (1): 13–25, Bibcode:2005PASJ...57...13T, doi:10.1093/pasj/57.1.13.
  4. "4 Equulei, a white main-sequence star in Equuleus", Ashland Astronomy Studio, archived from the original on 2013-04-14, retrieved 2012-08-19
  5. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  6. Ramírez, I.; et al. (September 2012), "Lithium Abundances in nearby FGK Dwarf and Subgiant Stars: Internal Destruction, Galactic Chemical Evolution, and Exoplanets", The Astrophysical Journal, 756 (1): 46, arXiv:1207.0499, Bibcode:2012ApJ...756...46R, doi:10.1088/0004-637X/756/1/46.
  7. "HIP 104101", Wolfram Alpha, retrieved 2012-08-19
  8. "4 Equ". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-02-18.
  9. Griffin, R. F. (October 2011), "Spectroscopic binary orbits from photoelectric radial velocities - Paper 220: 60 Piscium, 27 Arietis, EZ Ursae Majoris, and 4 Equulei", The Observatory, 131 (5): 294−314, Bibcode:2011Obs...131..294G.
  10. Where a is the actual semimajor axis and i is the orbital inclination.
  11. Israelian, G.; Santos, N. C.; Mayor, M.; Rebolo, R. (July 2003), "New measurement of the 6Li/7Li isotopic ratio in the extra-solar planet host star HD 82943 and line blending in the Li 6708 Å region", Astronomy and Astrophysics, 405: 753−762, arXiv:astro-ph/0304358, Bibcode:2003A&A...405..753I, doi:10.1051/0004-6361:20030591.
  12. Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122: 3466–3471, Bibcode:2001AJ....122.3466M, doi:10.1086/323920
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