31 Vulpeculae

31 Vulpeculae is a binary star[7] system in the northern constellation of Vulpecula. It is visible to the naked eye as a faint, yellow-hued point of light with an apparent visual magnitude of 4.56.[2] The system is located approximately 228 light years away from the Sun based on parallax,[1] and it is drifting further away with a radial velocity of +2.25 km/s.[5]

31 Vulpeculae
Location of 31 Vulpeculae (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Vulpecula
Right ascension 20h 52m 07.68352s[1]
Declination 27° 05′ 48.9951″[1]
Apparent magnitude (V) 4.56[2]
Characteristics
Evolutionary stage horizontal branch
Spectral type G7IIIa Fe-1 Ba[2]
U−B color index +0.46[3]
B−V color index +0.82[3]
Variable type suspected[4]
Astrometry
Radial velocity (Rv)+2.25[5] km/s
Proper motion (μ) RA: −70.902[1] mas/yr
Dec.: −56.360[1] mas/yr
Parallax (π)14.2796 ± 0.1864 mas[1]
Distance228 ± 3 ly
(70.0 ± 0.9 pc)
Absolute magnitude (MV)0.77[6]
Orbit[7]
Period (P)1,860.6±1.3 d
Semi-major axis (a)≥ 103.0 Gm
Eccentricity (e)0.375±0.009
Periastron epoch (T)52,316±6 MJD
Argument of periastron (ω)
(secondary)		15.1±1.4°
Semi-amplitude (K1)
(primary)		4.34±0.04 km/s
Details[8]
31 Vul A
Mass2.40±0.05 M☉
Radius8.01±0.30 R☉
Luminosity52.53[2] L☉
Surface gravity (log g)2.97±0.05 cgs
Temperature5,261±15 K
Metallicity [Fe/H]−0.05±0.02 dex
Rotational velocity (v sin i)5.9±0.3[7] km/s
Age700±40 Myr
Other designations
31 Vul, BD+26°4017, GC 29112, HD 198809, HIP 103004, HR 7995, SAO 89228, 2MASS J20520768+2705491[9]
Database references
SIMBADdata

The variable radial velocity of this system was first suspected by German Astronomer Friedrich Küstner in 1914. The system appears as a single-lined spectroscopic binary with an orbital period of 5.1 years and an eccentricity of 0.38. The a sin i value for the primary is 103.0 ± 1.1 Gm (0.69 ± 0.01 AU),[7] where a is the semimajor axis and i is the (unknown) orbital inclination. This value provides a lower bound on the actual semimajor axis for the orbit.

The primary component is an aging giant star with a stellar classification of G7IIIa Fe-1 Ba,[2] indicating a mild barium star with an underabundance of iron.[7] Having exhausted the supply of hydrogen at its core, it has expanded to eight times the Sun's radius.[8] It is a red clump giant,[10] which indicates it is on the horizontal branch and is generating energy through helium fusion at its core. This is a suspected variable star with a magnitude that varies from 3.77 to 4.08 in the I passband.[4] It is about 700 million years old with 2.4 times the mass of the Sun. The star is radiating 53[2] times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 5,261 K.[8] The companion may be a degenerate white dwarf with about 0.4 M☉.[7]

References
  1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. 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. S2CID 119257644. Vizier catalog entry
  3. Mallama, A. (2014). "Sloan Magnitudes for the Brightest Stars". The Journal of the American Association of Variable Star Observers. 42 (2): 443. Bibcode:2014JAVSO..42..443M.Vizier catalog entry
  4. Samus, N. N.; Durlevich, O. V.; et al. (2009). "General Catalogue of Variable Stars". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  5. Soubiran, C.; Bienaymé, O.; Mishenina, T. V.; Kovtyukh, V. V. (2008). "Vertical distribution of Galactic disk stars". Astronomy and Astrophysics. 480 (1): 91–101. arXiv:0712.1370. Bibcode:2008A&A...480...91S. doi:10.1051/0004-6361:20078788. S2CID 16602121.
  6. Da Silva, Ronaldo; et al. (2015). "Homogeneous abundance analysis of FGK dwarf, subgiant, and giant stars with and without giant planets". Astronomy & Astrophysics. 580: A24. arXiv:1505.01726. Bibcode:2015A&A...580A..24D. doi:10.1051/0004-6361/201525770. S2CID 119216425. Vizier catalog entry
  7. Griffin, R. F. (August 2009). "Spectroscopic binary orbits from photoelectric radial velocities. Paper 207: 58 Piscium, 31 Vulpeculae, and 70 Pegasi". The Observatory. 129: 198–218. Bibcode:2009Obs...129..198G.
  8. Maldonado, J.; Villaver, E. (April 2016). "Evolved stars and the origin of abundance trends in planet hosts". Astronomy & Astrophysics. 588: 11. arXiv:1602.00835. Bibcode:2016A&A...588A..98M. doi:10.1051/0004-6361/201527883. S2CID 119212009. A98.
  9. "31 Vul". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-07-13.
  10. Alves, David R. (August 2000), "K-Band Calibration of the Red Clump Luminosity", The Astrophysical Journal, 539 (2): 732–741, arXiv:astro-ph/0003329, Bibcode:2000ApJ...539..732A, doi:10.1086/309278, S2CID 16673121
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