{{Short description|Likely binary star in the constellation Draco}} {{Starbox begin}} {{Starbox image | image = {{Location mark |image=Draco constellation map.svg|alt=|float=center|width=280 |label=|position=right |mark=Red circle.svg|mark_width=10|mark_link=HD 81817 |x=690|y=117 }} | caption = Location of HD 81817 (circled in red) }} {{Starbox observe | constell = [[Draco (constellation)|Draco]]<ref name=Anderson2012/> | epoch = J2000.0 | ra = {{RA|09|37|05.28778}}<ref name=vanLeeuwen2007/> | dec = {{DEC|+81|19|34.9711}}<ref name=vanLeeuwen2007/> | appmag_v = 4.28<ref name=Anderson2012/> }} {{Starbox character | type = | class = K3III + ? + DA2.8<ref name=Holberg2013/> | b-v = {{Val|+1.488|0.001}}<ref name=Anderson2012/> | u-b = | variable = }} {{Starbox astrometry | radial_v = {{Val|−6.98|0.10}}<ref name=Anderson2012/> | prop_mo_ra = −16.62<ref name=vanLeeuwen2007/> | prop_mo_dec = −16.41<ref name=vanLeeuwen2007/> | parallax = 3.28 | p_error = 0.15 | parallax_footnote = <ref name=vanLeeuwen2007/> | absmag_v = −3.14<ref name=Anderson2012/> }} <!--{{Starbox orbit | reference = <ref name=Bang2020/> | name = HD 81817 Ab | period_unitless = {{Val|1047.1|8.5|ul=d}} | axis_unitless = {{Val|3.3|0.1|ul=AU}} | eccentricity = {{Val|0.17|0.07}} | periastron = {{Val|2449712|108}} | periarg = {{Val|320|34}} }}--> {{Starbox detail | source = <ref name=Bang2020/> | component1 = A | mass = {{Val|4.3|0.5}} | radius = {{Val|83.8|7.8}} | luminosity = 1,822.9 | gravity = | temperature = {{Val|4140|55|fmt=commas}} | metal_fe = {{Val|-0.17|0.1}} | rotational_velocity = {{val|4.7|0.1}} | rotation = {{val|801|ul=days}} | age_myr = {{Val|150|40}} }} {{Starbox catalog | names = {{odlist | BD=+81°302 | FK5=910 | HD=81817 | HIP=47193 | HR=3751 | SAO=1551 | WD=0930+815 }}<ref name=SIMBAD/> }} {{Starbox reference | Simbad = HD+81817 | NSTED = HD-81817 }} {{Starbox end}}
'''HD 81817''' is a possible [[binary star]] system<ref name=Reimers1984/> with two [[brown dwarf]] companions<ref name=Bang2020/><ref name="Feng2022"/> in the northern [[circumpolar constellation]] of [[Draco (constellation)|Draco]]. It has an orange hue and is visible to the naked eye with an [[apparent visual magnitude]] of 4.28.<ref name=Anderson2012/> The system is located at a distance of approximately 990 [[light year]]s from the [[Sun]] based on [[stellar parallax|parallax]],<ref name=vanLeeuwen2007/> and is drifting closer with a [[radial velocity]] of −7 km/s.<ref name=Anderson2012/> It is a member of the [[IC 2391]] moving group.<ref name=Montes2001/>
The primary component of this system is an aging [[giant star]] with a [[stellar classification]] of K3III.<ref name=Holberg2013/> The star's [[chromosphere]] is of the type called "hybrid", displaying a cool stellar wind in combination with hot emission lines.<ref name=Ayres2005/> It appears to be the source for the [[X-ray astronomy|X-ray emission]] coming from this system.<ref name=Bilíková2010/> The star is 150 million years old with 4.3 times the [[mass of the Sun]].<ref name=Bang2020/> With the supply of hydrogen at its [[stellar core|core]] exhausted, the star has expanded to 83.8 times the [[Sun's radius]].<ref name=Bang2020/> It is radiating 1,823 times the [[luminosity of the Sun]] from its [[photosphere]] at an [[effective temperature]] of {{val|4140|ul=K|fmt=commas}}.<ref name=Bang2020/>
A possible companion star was discovered in 1984 based upon its ultraviolet spectrum. The distribution of the far ultraviolet flux matches that of a [[white dwarf]] star of class DA.<ref name=Reimers1984/> A 2020 study finds it unlikely that there is a white dwarf companion; instead claiming the [[radial velocity]] variations are caused by a substellar object HD 81817 b, probably a [[brown dwarf]], and possibly another substellar object. HD 81817 b would have a [[minimum mass]] of {{Jupiter mass|27.1|link=y}} and orbit at 3.3 [[astronomical unit|AU]] with a period of 1047.1 days and an [[orbital eccentricity|eccentricity]] of 0.17. [[Gaia (spacecraft)|Gaia DR2]] [[astrometry]] also suggested a companion with a mass of about {{Jupiter mass|124|link=y}} (with a high margin of error) orbiting at 2.67 AU, consistent with the radial velocity detection. If the latter mass estimate is correct, this object would be a low-mass star, probably a [[red dwarf]].<ref name=Bang2020/> Further observations through 2022 have confirmed that the companion is indeed a brown dwarf, in addition to the detection of a second brown dwarf on a closer orbit.<ref name="Feng2022"/>
{{Orbitbox planet begin | name = HD 81817 | table_ref = <ref name="Feng2022"/> | period_unit = year }} {{Orbitbox planet | exoplanet = c | mass = {{val|22.609|1.859|1.876|p=≥}} | radius = | semimajor = {{val|2.325|0.087|0.095}} | period = {{val|1.706|0.006}} | eccentricity = {{val|0.095|0.058|0.044}} | inclination = }} {{Orbitbox planet | exoplanet = b | mass = {{val|24.128|9.747|0.691}} | radius = | semimajor = {{val|3.233|0.125|0.131}} | period = {{val|2.796|0.037|0.027}} | eccentricity = {{val|0.097|0.090|0.057}} | inclination = {{val|95.594|29.637|35.307}} }} {{Orbitbox end}}
== See also ==
* [[List of exoplanets discovered in 2020]] - HD 81817 b
== References == {{Reflist|refs=
<!--<ref name=GaiaDR3>{{Cite Gaia DR3|1144716265942885888}}</ref>-->
<ref name=SIMBAD>{{cite simbad | title=HD 81817 | access-date=2020-01-26 }}</ref>
<ref name=Anderson2012>{{cite XHIP|47193}}</ref>
<ref name=vanLeeuwen2007>{{cite journal | last1=van Leeuwen | first1=F. | title=Validation of the new Hipparcos reduction | journal=Astronomy and Astrophysics | arxiv=0708.1752 | volume=474 | issue=2 | pages=653–664 | year=2007 | bibcode=2007A&A...474..653V | bibcode-access=free | doi=10.1051/0004-6361:20078357 | doi-access=free }}</ref>
<ref name=Holberg2013>{{cite journal | display-authors=1 | last1=Holberg | first1=J. B. | last2=Oswalt | first2=T. D. | last3=Sion | first3=E. M. | last4=Barstow | first4=M. A. | last5=Burleigh | first5=M. R. | title=Where are all the Sirius-like binary systems? | journal=Monthly Notices of the Royal Astronomical Society | volume=435 | issue=3 | pages=2077 | year=2013 | doi=10.1093/mnras/stt1433 | doi-access=free | arxiv=1307.8047 | bibcode=2013MNRAS.435.2077H }}</ref>
<ref name=Reimers1984>{{cite journal | title=Discovery of a white dwarf companion of the "hybrid" K giant HD 81817 | last=Reimers | first=D. | journal=Astronomy and Astrophysics | volume=136 | pages=L5–L6 | date=July 1984 | bibcode=1984A&A...136L...5R }}</ref>
<!--<ref name=Tetzlaff2011>{{cite journal | last1=Tetzlaff | first1=N. | last2=Neuhäuser | first2=R. | last3=Hohle | first3=M. M. | title=A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun | journal=Monthly Notices of the Royal Astronomical Society | volume=410 | issue=1 | pages=190–200 | date=January 2011 | bibcode=2011MNRAS.410..190T | arxiv=1007.4883 | doi=10.1111/j.1365-2966.2010.17434.x | s2cid=118629873 }}</ref>-->
<ref name=Montes2001>{{cite journal | last1=Montes | first1=D. | last2=López-Santiago | first2=J. | last3=Gálvez | first3=M. C. | last4=Fernández-Figueroa | first4=M. J. | last5=De Castro | first5=E. | last6=Cornide | first6=M. | title=Late-type members of young stellar kinematic groups - I. Single stars | display-authors=1 | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=328 | issue=1 | pages=45–63 | date=November 2001 | bibcode=2001MNRAS.328...45M | arxiv=astro-ph/0106537 | doi=10.1046/j.1365-8711.2001.04781.x | doi-access=free | s2cid=55727428 }}</ref>
<ref name=Ayres2005>{{cite journal | title=X-Rays from Hybrid Stars | last=Ayres | first=Thomas R. | journal=The Astrophysical Journal | volume=618 | issue=1 | pages=493–501 | date=January 2005 | doi=10.1086/425891 | bibcode=2005ApJ...618..493A | s2cid=123363660 | doi-access=free }}</ref>
<ref name=Bilíková2010>{{cite journal | title=Hard X-ray Emission Associated with White Dwarfs. III. | last1=Bilíková | first1=Jana | last2=Chu | first2=You-Hua | last3=Gruendl | first3=Robert A. | last4=Maddox | first4=Larry A. | display-authors=1 | journal=The Astronomical Journal | volume=140 | issue=5 | pages=1433–1443 | date=November 2010 | doi=10.1088/0004-6256/140/5/1433 | bibcode=2010AJ....140.1433B | doi-access= }}</ref>
<ref name=Bang2020>{{cite journal | title=Hybrid star HD 81817 accompanied by brown dwarf or substellar companion | last1=Bang | first1=Tae-Yang | last2=Lee | first2=Byeong-Cheol | last3=Perdelwitz | first3=Volker | last4=Jeong | first4=Gwanghui | last5=Han | first5=Inwoo | last6=Oh | first6=Hyeong-Il | last7=Park | last8=Myeong-Gu | display-authors=1 | journal=[[Astronomy & Astrophysics]] | volume=638 | pages=A148 | date=June 2020 | doi=10.1051/0004-6361/201936613 | bibcode=2020A&A...638A.148B | doi-access=free |arxiv=2005.09273 }}</ref>
<ref name="Feng2022">{{cite journal |last1=Feng |first1=Fabo |last2=Butler |first2=R. Paul |display-authors=etal |date=August 2022 |title=3D Selection of 167 Substellar Companions to Nearby Stars |journal=[[The Astrophysical Journal Supplement Series]] |volume=262 |issue=21 |page=21 |doi=10.3847/1538-4365/ac7e57 |arxiv=2208.12720 |bibcode=2022ApJS..262...21F|s2cid=251864022 |doi-access=free }}</ref>
}}
{{Stars of Draco}}
<!-- Properties--> [[Category:K-type giants]] [[Category:White dwarfs]] [[Category:Spectroscopic binaries]] <!-- Identifiers --> [[Category:Draco (constellation)]] [[Category:Durchmusterung objects|BD+81 0302]] [[Category:Henry Draper Catalogue objects|081817]] [[Category:Hipparcos objects|047193]] [[Category:Bright Star Catalogue objects|3751]] [[Category:Planetary systems with two confirmed planets]] [[Category:Brown dwarfs]]