View source for HR 6806

{{Short description|Star in the constellation Hercules}}
{{Starbox begin
 | name=HR 6806
}}
{{Starbox observe
 | epoch=J2000
 | constell=[[Hercules (constellation)|Hercules]]
 | ra={{RA|18|09|37.41628}}<ref name=GaiaDR2/>
 | dec={{DEC|+38|27|27.9959}}<ref name=GaiaDR2/>
 | appmag_v=6.40<ref name="MERMILLIOD"/>
}}
{{Starbox character
 | class=K2&nbsp;V<ref name="Gray"/>
 | r-i=
 | v-r=
 | b-v=+0.875<ref name="MERMILLIOD"/>
 | u-b=+0.585<ref name="MERMILLIOD"/>
 | variable=
}}
{{Starbox astrometry
 | radial_v={{Val|−19.315|0.0015}}<ref name=GaiaDR2/>
 | prop_mo_ra=−316.520<ref name=GaiaDR2/>
 | prop_mo_dec=−468.214<ref name=GaiaDR2/>
 | parallax=90.1264
 | p_error=0.0200
 | parallax_footnote=<ref name=GaiaDR2/>
 | absmag_v=6.17<ref name=Holmberg2009/>
}}
{{Starbox detail
 | mass={{Val|0.791|0.014|0.008}}<ref name=Marsden2014/>
 | radius={{Val|0.79|0.02|0.01}}<ref name=Marsden2014/>
 | luminosity=0.35<ref name=Marsden2014/>
 | temperature=4,900<ref name="Gray"/>
 | metal_fe=−0.61<ref name="Gray"/>
 | rotation=42&nbsp;d<ref name=apj1_330_274/>
 | rotational_velocity=4.82<ref name="MartínezArnáiz2010"/>
 | gravity=4.53<ref name="Gray"/>
 | age_gyr=5.8–7.1<ref name="apj687"/>
}}
{{Starbox catalog
 | names={{odlist | BD=+38°3095 | GJ=706 | HD=166620 | HIP=88972 | HR=6806 | SAO=66700 | LHS=3363 }}<ref name=SIMBAD/>
}}
{{Starbox reference
 | Simbad = HD+166620
}}
{{Starbox end}}

'''HR 6806''' or '''HD 166620''' is a solitary, orange, [[main sequence]], and [[Sun-like]] (K2 V) star located thirty-six light-years away,<ref name="WrightIsaacson2022"/> in the constellation [[Hercules (constellation)|Hercules]]. The star is smaller than the Sun, with around 79% of the solar mass and radius, and 35% of the solar luminosity.<ref name=Marsden2014/> It appears to be rotating slowly with an estimated period of 42&nbsp;days. In 1988, it was noticed that the star had an inactive [[chromosphere]], with a surface [[stellar magnetic field|magnetic field]] strength of only 1,500&nbsp;[[Gauss (unit)|G]].<ref name=apj1_330_274/> 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|post=,}} the star appears to have entered the equivalent of a [[Maunder minimum]].<ref name="WrightIsaacson2022"/> The star is around six billion years of age.<ref name="apj687"/>

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&nbsp;[[Astronomical Unit|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 {{Convert|28|pc|ly|abbr=on|lk=off|order=flip}}—ruling out a physical association. This is confirmed by the differing proper motion of the star and this object.<ref name=apj760_2_152/>

== References ==
{{reflist|refs=

<ref name=SIMBAD>{{cite simbad | title=HD 166620 | access-date=2019-12-07 }}</ref>

<ref name=GaiaDR2>{{cite DR2|4610177733290768896}}</ref>

<ref name=Holmberg2009>{{citation
 | display-authors=1 | last1=Holmberg | first1=J.
 | last2=Nordström | first2=B. | last3=Andersen | first3=J.
 | title=The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
 | volume=501 | issue=3 |date=July 2009 | pages=941–947
 | journal=Astronomy and Astrophysics
 | doi=10.1051/0004-6361/200811191 | bibcode=2009A&A...501..941H
 | arxiv=0811.3982 | s2cid=118577511 | postscript=. }}</ref>

<ref name="apj687">{{citation
 | last1=Mamajek | first1=Eric E.
 | last2=Hillenbrand | first2=Lynne A.
 | title=Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
 | journal=The Astrophysical Journal
 | volume=687 | issue=2 | pages=1264–1293 |date=November 2008
 | doi=10.1086/591785
 | bibcode=2008ApJ...687.1264M
 | arxiv = 0807.1686
 | s2cid=27151456
 | postscript=.
}}</ref>

<ref name="Gray">{{citation
 | display-authors=1
 | last1=Gray | first1=R. O.
 | last2=Corbally | first2=C. J.
 | last3=Garrison | first3=R. F.
 | last4=McFadden | first4=M. T.
 | last5=Bubar | first5=E. J. | last6=McGahee | first6=C. E.
 | last7=O'Donoghue | first7=A. A.
 | last8=Knox | first8=E. R.
 | title=Contributions to the Nearby Stars (NStars) Project: spectroscopy of stars earlier than M0 within 40 pc-The Southern Sample
 | journal=[[The Astronomical Journal]]
 | volume=132 | issue=1 | pages=161–170 | date=July 2006
 | doi=10.1086/504637
 | bibcode=2006AJ....132..161G
 | arxiv=astro-ph/0603770
 | s2cid=119476992 | postscript=.
}}</ref>

<ref name="MartínezArnáiz2010">{{citation
 |display-authors = 1
 |last1           = Martínez-Arnáiz
 |first1          = R.
 |last2           = Maldonado
 |first2          = J.
 |last3           = Montes
 |first3          = D.
 |last4           = Eiroa
 |first4          = C.
 |last5           = Montesinos
 |first5          = B.
 |title           = Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter
 |journal         = Astronomy and Astrophysics
 |volume          = 520
 |page            = A79
 |date            = September 2010
 |doi             = 10.1051/0004-6361/200913725
 |bibcode         = 2010A&A...520A..79M
 |arxiv           = 1002.4391
 |s2cid           = 43455849
 |postscript      = .
 |url             = http://eprints.ucm.es/37826/1/davidmontes17libre.pdf
 |access-date     = 2018-11-04
 |archive-date    = 2017-09-22
 |archive-url     = https://web.archive.org/web/20170922031257/http://eprints.ucm.es/37826/1/davidmontes17libre.pdf
 |url-status      = dead
}}</ref>

<ref name=apj1_330_274>{{citation
 | last1=Basri | first1=Gibor
 | last2=Marcy | first2=Geoffrey W. 
 | title=Physical realism in the analysis of stellar magnetic fields 
 | journal=Astrophysical Journal, Part 1
 | volume=330 | pages=274–285 | date=July 1988
 | doi=10.1086/166471 
 | bibcode=1988ApJ...330..274B 
 | postscript=.
}}</ref>

<ref name=apj760_2_152>{{citation
 | display-authors=1
 | last1=Luhman | first1=Kevin L.
 | last2=Loutrel | first2=Nicholas P.
 | last3=McCurdy | first3=Nicholas S.
 | last4=Mace | first4=Gregory N.
 | last5=Melso | first5=Nicole D.
 | last6=Star | first6=Kimberly M.
 | last7=Young | first7=Michael D.
 | last8=Terrien | first8=Ryan C.
 | last9=McLean | first9=Ian S.
 | last10=Kirkpatrick | first10=J. Davy
 | last11=Rhode | first11=Katherine L. 
 | title=New M, L, and T Dwarf Companions to Nearby Stars from the Wide-field Infrared Survey Explorer 
 | journal=The Astrophysical Journal
 | volume=760 | issue=2 | id=152 | pages=9
 | date=December 2012
 | doi=10.1088/0004-637X/760/2/152 
 | bibcode=2012ApJ...760..152L
 | arxiv=1211.3977
 | s2cid=51010785 | postscript=.
}}</ref>

<ref name="MERMILLIOD">{{citation
 | last=Mermilliod | first=J.-C.
 | title=Compilation of Eggen's UBV data, transformed to UBV (unpublished)
 | journal=Catalogue of Eggen's UBV Data
 | date=1986
 | publisher=[[SIMBAD]]
 | bibcode=1986EgUBV........0M
 | postscript=.
}}</ref>

<ref name=Marsden2014>{{citation
 | display-authors=1
 | last1=Marsden | first1=S. C. | last2=Petit | first2=P.
 | last3=Jeffers | first3=S. V. | last4=Morin | first4=J.
 | last5=Fares | first5=R. | last6=Reiners | first6=A.
 | last7=do Nascimento | first7=J.-D. | last8=Aurière | first8=M.
 | last9=Bouvier | first9=J. | last10=Carter | first10=B. D.
 | last11=Catala | first11=C. | last12=Dintrans | first12=B.
 | last13=Donati | first13=J.-F. | last14=Gastine | first14=T.
 | last15=Jardine | first15=M. | last16=Konstantinova-Antova | first16=R.
 | last17=Lanoux | first17=J. | last18=Lignières | first18=F.
 | last19=Morgenthaler | first19=A. | last20=Ramìrez-Vèlez | first20=J. C.
 | last21=Théado | first21=S. | last22=Van Grootel | first22=V.
 | last23=BCool Collaboration
 | title=A BCool magnetic snapshot survey of solar-type stars
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=444 | issue=4 | pages=3517–3536 | date=November 2014
 | doi=10.1093/mnras/stu1663 | doi-access=free | bibcode=2014MNRAS.444.3517M
 | arxiv=1311.3374 | postscript=. }}</ref>

<ref name="WrightIsaacson2022">{{cite journal|last1=Baum|first1=Anna C.|last2=Wright|first2=Jason T.|last3=Luhn|first3=Jacob K.|last4=Isaacson|first4=Howard|display-authors=2|title=Five Decades of Chromospheric Activity in 59 Sun-like Stars and New Maunder Minimum Candidate HD 166620|journal=The Astronomical Journal|publisher=American Astronomical Society|volume=163|issue=4|date=March 22, 2022|issn=0004-6256|doi=10.3847/1538-3881/ac5683|doi-access=free|page=183|arxiv=2203.13376 |bibcode=2022AJ....163..183B }}</ref>

}}

== External links ==
* {{ARICNS link|id=4c01473|label='''ARICNS''': 01473}}
* [http://nstars.nau.edu/index.cfm?fuseaction=singleSearch.systemID&SystemID=1809p3827 '''NStars''': 1809+3827]{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}

{{Stars of Hercules}}

{{DEFAULTSORT:HR 6806}}
[[Category:K-type main-sequence stars]]
[[Category:Solar-type stars|HR, 6806]]
[[Category:Maunder Minimum]]
[[Category:Hercules (constellation)]]
[[Category:Durchmusterung objects]]
[[Category:Gliese and GJ objects|0706]]
[[Category:Henry Draper Catalogue objects|166620]]
[[Category:Hipparcos objects|088972]]
[[Category:Bright Star Catalogue objects|6806]]