HR 6806

HR 6806 or HD 166620 is a solitary, orange, main sequence, and Sun-like (K2 V) star located thirty-six light-years away,[10] in the constellation Hercules. The star is smaller than the Sun, with around 79% of the solar mass and radius, and 35% of the solar luminosity.[5] It appears to be rotating slowly with an estimated period of 42 days. In 1988, it was noticed that the star had an inactive chromosphere, with a surface magnetic field strength of only 1,500 G.[6] From 1990 activity in the chromosphere increased, inline with a 16 year stellar cycle previously observed. But, sometime after 1994 (exact date unknown because of a data collection gap between 1995 and 2004) chromospheric activity greatly reduced, and has stayed flat for more than 16 years. As of 2022, the star appears to have entered the equivalent of a Maunder minimum.[10] The star is around six billion years of age.[8]

HR 6806
Observation data
Epoch J2000      Equinox J2000
Constellation Hercules
Right ascension 18h 09m 37.41628s[1]
Declination +38° 27′ 27.9959″[1]
Apparent magnitude (V) 6.40[2]
Characteristics
Spectral type K2 V[3]
U−B color index +0.585[2]
B−V color index +0.875[2]
Astrometry
Radial velocity (Rv)−19.315±0.0015[1] km/s
Proper motion (μ) RA: −316.520[1] mas/yr
Dec.: −468.214[1] mas/yr
Parallax (π)90.1264 ± 0.0200 mas[1]
Distance36.189 ± 0.008 ly
(11.096 ± 0.002 pc)
Absolute magnitude (MV)6.17[4]
Details
Mass0.791+0.014
−0.008[5] M☉
Radius0.79+0.02
−0.01[5] R☉
Luminosity0.35[5] L☉
Surface gravity (log g)4.53[3] cgs
Temperature4,900[3] K
Metallicity [Fe/H]−0.61[3] dex
Rotation42 d[6]
Rotational velocity (v sin i)4.82[7] km/s
Age5.8–7.1[8] Gyr
Other designations
BD+38°3095, GJ 706, HD 166620, HIP 88972, HR 6806, SAO 66700, LHS 3363[9]
Database references
SIMBADdata

There was suspected to be a nearby very cool, and very dim, T9 to Y brown dwarf companion, WISE J180901.07+383805.4, at an angular separation of 769″, which would have corresponded to a projected separation of 8460 AU at the distance of HR 6806. However, with further observation it was found to be bluer than at first thought and more typical of a slightly brighter T7 dwarf, which would place it at a much greater distance of 91 ly (28 pc)—ruling out a physical association. This is confirmed by the differing proper motion of the star and this object.[11]

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. Mermilliod, J.-C. (1986), "Compilation of Eggen's UBV data, transformed to UBV (unpublished)", Catalogue of Eggen's UBV Data, SIMBAD, Bibcode:1986EgUBV........0M.
  3. Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637, S2CID 119476992.
  4. Holmberg, J.; et al. (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, S2CID 118577511.
  5. Marsden, S. C.; et al. (November 2014), "A BCool magnetic snapshot survey of solar-type stars", Monthly Notices of the Royal Astronomical Society, 444 (4): 3517–3536, arXiv:1311.3374, Bibcode:2014MNRAS.444.3517M, doi:10.1093/mnras/stu1663.
  6. Basri, Gibor; Marcy, Geoffrey W. (July 1988), "Physical realism in the analysis of stellar magnetic fields", Astrophysical Journal, Part 1, 330: 274–285, Bibcode:1988ApJ...330..274B, doi:10.1086/166471.
  7. Martínez-Arnáiz, R.; et al. (September 2010), "Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter" (PDF), Astronomy and Astrophysics, 520: A79, arXiv:1002.4391, Bibcode:2010A&A...520A..79M, doi:10.1051/0004-6361/200913725, S2CID 43455849, archived from the original (PDF) on 2017-09-22, retrieved 2018-11-04.
  8. Mamajek, Eric E.; Hillenbrand, Lynne A. (November 2008), "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics", The Astrophysical Journal, 687 (2): 1264–1293, arXiv:0807.1686, Bibcode:2008ApJ...687.1264M, doi:10.1086/591785, S2CID 27151456.
  9. "HD 166620". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-12-07.
  10. Baum, Anna C.; Wright, Jason T.; et al. (March 22, 2022). "Five Decades of Chromospheric Activity in 59 Sun-like Stars and New Maunder Minimum Candidate HD 166620". The Astronomical Journal. American Astronomical Society. 163 (4): 183. arXiv:2203.13376. Bibcode:2022AJ....163..183B. doi:10.3847/1538-3881/ac5683. ISSN 0004-6256.
  11. Luhman, Kevin L.; et al. (December 2012), "New M, L, and T Dwarf Companions to Nearby Stars from the Wide-field Infrared Survey Explorer", The Astrophysical Journal, 760 (2): 9, arXiv:1211.3977, Bibcode:2012ApJ...760..152L, doi:10.1088/0004-637X/760/2/152, S2CID 51010785, 152.
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