# HD 81817

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Likely binary star in the constellation Draco

HD 81817 Location of HD 81817 (circled in red) Observation data Epoch J2000.0 Equinox J2000.0 Constellation Draco[1] Right ascension 09h 37m 05.28778s[2] Declination +81° 19′ 34.9711″[2] Apparent magnitude (V) 4.28[1] Characteristics Spectral type K3III + ? + DA2.8[3] B−V color index +1.488±0.001[1] Astrometry Radial velocity (Rv) −6.98±0.10[1] km/s Proper motion (μ) RA: −16.62[2] mas/yr Dec.: −16.41[2] mas/yr Parallax (π) 3.28±0.15 mas[2] Distance 990 ± 50 ly (300 ± 10 pc) Absolute magnitude (MV) −3.14[1] Details[4] A Mass 4.3±0.5 M☉ Radius 83.8±7.8 R☉ Luminosity 1,822.9 L☉ Temperature 4,140±55 K Metallicity [Fe/H] −0.17±0.1 dex Rotation 801 d Rotational velocity (v sin i) 4.7±0.1 km/s Age 150±40 Myr Other designations BD+81°302, FK5 910, HD 81817, HIP 47193, HR 3751, SAO 1551, WD 0930+815[5] Database references SIMBAD data Exoplanet Archive data

**HD 81817** is a possible [binary star](/source/Binary_star) system[6] with two [brown dwarf](/source/Brown_dwarf) companions[4][7] in the northern [circumpolar constellation](/source/Circumpolar_constellation) of [Draco](/source/Draco_(constellation)). It has an orange hue and is visible to the naked eye with an [apparent visual magnitude](/source/Apparent_visual_magnitude) of 4.28.[1] The system is located at a distance of approximately 990 [light years](/source/Light_year) from the [Sun](/source/Sun) based on [parallax](/source/Stellar_parallax),[2] and is drifting closer with a [radial velocity](/source/Radial_velocity) of −7 km/s.[1] It is a member of the [IC 2391](/source/IC_2391) moving group.[8]

The primary component of this system is an aging [giant star](/source/Giant_star) with a [stellar classification](/source/Stellar_classification) of K3III.[3] The star's [chromosphere](/source/Chromosphere) is of the type called "hybrid", displaying a cool stellar wind in combination with hot emission lines.[9] It appears to be the source for the [X-ray emission](/source/X-ray_astronomy) coming from this system.[10] The star is 150 million years old with 4.3 times the [mass of the Sun](/source/Mass_of_the_Sun).[4] With the supply of hydrogen at its [core](/source/Stellar_core) exhausted, the star has expanded to 83.8 times the [Sun's radius](/source/Sun's_radius).[4] It is radiating 1,823 times the [luminosity of the Sun](/source/Luminosity_of_the_Sun) from its [photosphere](/source/Photosphere) at an [effective temperature](/source/Effective_temperature) of 4,140 [K](/source/Kelvin).[4]

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](/source/White_dwarf) star of class DA.[6] A 2020 study finds it unlikely that there is a white dwarf companion; instead claiming the [radial velocity](/source/Radial_velocity) variations are caused by a substellar object HD 81817 b, probably a [brown dwarf](/source/Brown_dwarf), and possibly another substellar object. HD 81817 b would have a [minimum mass](/source/Minimum_mass) of 27.1 [MJ](/source/Jupiter_mass) and orbit at 3.3 [AU](/source/Astronomical_unit) with a period of 1047.1 days and an [eccentricity](/source/Orbital_eccentricity) of 0.17. [Gaia DR2](/source/Gaia_(spacecraft)) [astrometry](/source/Astrometry) also suggested a companion with a mass of about 124 [MJ](/source/Jupiter_mass) (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](/source/Red_dwarf).[4] 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.[7]

The HD 81817 planetary system[7] Companion (in order from star) Mass Semimajor axis (AU) Orbital period (years) Eccentricity Inclination (°) Radius c ≥22.609+1.859 −1.876 MJ 2.325+0.087 −0.095 1.706±0.006 0.095+0.058 −0.044 — — b 24.128+9.747 −0.691 MJ 3.233+0.125 −0.131 2.796+0.037 −0.027 0.097+0.090 −0.057 95.594+29.637 −35.307 —

## See also

- [List of exoplanets discovered in 2020](/source/List_of_exoplanets_discovered_in_2020) - HD 81817 b

## References

1. ^ [***a***](#cite_ref-Anderson2012_1-0) [***b***](#cite_ref-Anderson2012_1-1) [***c***](#cite_ref-Anderson2012_1-2) [***d***](#cite_ref-Anderson2012_1-3) [***e***](#cite_ref-Anderson2012_1-4) [***f***](#cite_ref-Anderson2012_1-5) [***g***](#cite_ref-Anderson2012_1-6) Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". *[Astronomy Letters](/source/Astronomy_Letters)*. **38** (5): 331. [arXiv](/source/ArXiv_(identifier)):[1108.4971](https://arxiv.org/abs/1108.4971). [Bibcode](/source/Bibcode_(identifier)):[2012AstL...38..331A](https://ui.adsabs.harvard.edu/abs/2012AstL...38..331A). [doi](/source/Doi_(identifier)):[10.1134/S1063773712050015](https://doi.org/10.1134%2FS1063773712050015). [XHIP record for this object](https://vizier.cds.unistra.fr/viz-bin/VizieR-4?-ref=VIZ681510a8389ecb&-out.src=V%2F137D%2FXHIP&-nav=cat%3AV%2F137D%26tab%3A{V%2F137D%2FXHIP}%26key%3Asource%3DV%2F137D%2FXHIP%26HTTPPRM%3A&-source=V%2F137D%2FXHIP&-out=HIP&HIP=47193&-out=Comp&-out=Gr&-out=RAJ2000&-out=DEJ2000&-out=Dist&-out=X&-out=Y&-out=Z&-out=SpType&-out=Simbad&-out=Hpmag&-out=Lum&-out=Cst) at [VizieR](/source/VizieR).

1. ^ [***a***](#cite_ref-vanLeeuwen2007_2-0) [***b***](#cite_ref-vanLeeuwen2007_2-1) [***c***](#cite_ref-vanLeeuwen2007_2-2) [***d***](#cite_ref-vanLeeuwen2007_2-3) [***e***](#cite_ref-vanLeeuwen2007_2-4) [***f***](#cite_ref-vanLeeuwen2007_2-5) van Leeuwen, F. (2007). ["Validation of the new Hipparcos reduction"](https://doi.org/10.1051%2F0004-6361%3A20078357). *Astronomy and Astrophysics*. **474** (2): 653–664. [arXiv](/source/ArXiv_(identifier)):[0708.1752](https://arxiv.org/abs/0708.1752). [Bibcode](/source/Bibcode_(identifier)):[2007A&A...474..653V](https://ui.adsabs.harvard.edu/abs/2007A&A...474..653V). [doi](/source/Doi_(identifier)):[10.1051/0004-6361:20078357](https://doi.org/10.1051%2F0004-6361%3A20078357).

1. ^ [***a***](#cite_ref-Holberg2013_3-0) [***b***](#cite_ref-Holberg2013_3-1) Holberg, J. B.; et al. (2013). ["Where are all the Sirius-like binary systems?"](https://doi.org/10.1093%2Fmnras%2Fstt1433). *Monthly Notices of the Royal Astronomical Society*. **435** (3): 2077. [arXiv](/source/ArXiv_(identifier)):[1307.8047](https://arxiv.org/abs/1307.8047). [Bibcode](/source/Bibcode_(identifier)):[2013MNRAS.435.2077H](https://ui.adsabs.harvard.edu/abs/2013MNRAS.435.2077H). [doi](/source/Doi_(identifier)):[10.1093/mnras/stt1433](https://doi.org/10.1093%2Fmnras%2Fstt1433).

1. ^ [***a***](#cite_ref-Bang2020_4-0) [***b***](#cite_ref-Bang2020_4-1) [***c***](#cite_ref-Bang2020_4-2) [***d***](#cite_ref-Bang2020_4-3) [***e***](#cite_ref-Bang2020_4-4) [***f***](#cite_ref-Bang2020_4-5) Bang, Tae-Yang; et al. (June 2020). ["Hybrid star HD 81817 accompanied by brown dwarf or substellar companion"](https://doi.org/10.1051%2F0004-6361%2F201936613). *[Astronomy & Astrophysics](/source/Astronomy_%26_Astrophysics)*. **638**: A148. [arXiv](/source/ArXiv_(identifier)):[2005.09273](https://arxiv.org/abs/2005.09273). [Bibcode](/source/Bibcode_(identifier)):[2020A&A...638A.148B](https://ui.adsabs.harvard.edu/abs/2020A&A...638A.148B). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201936613](https://doi.org/10.1051%2F0004-6361%2F201936613).

1. **[^](#cite_ref-SIMBAD_5-0)** ["HD 81817"](http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=HD+81817). *[SIMBAD](/source/SIMBAD)*. [Centre de données astronomiques de Strasbourg](/source/Centre_de_donn%C3%A9es_astronomiques_de_Strasbourg). Retrieved 2020-01-26.

1. ^ [***a***](#cite_ref-Reimers1984_6-0) [***b***](#cite_ref-Reimers1984_6-1) Reimers, D. (July 1984). "Discovery of a white dwarf companion of the "hybrid" K giant HD 81817". *Astronomy and Astrophysics*. **136**: L5–L6. [Bibcode](/source/Bibcode_(identifier)):[1984A&A...136L...5R](https://ui.adsabs.harvard.edu/abs/1984A&A...136L...5R).

1. ^ [***a***](#cite_ref-Feng2022_7-0) [***b***](#cite_ref-Feng2022_7-1) [***c***](#cite_ref-Feng2022_7-2) Feng, Fabo; Butler, R. Paul; et al. (August 2022). ["3D Selection of 167 Substellar Companions to Nearby Stars"](https://doi.org/10.3847%2F1538-4365%2Fac7e57). *[The Astrophysical Journal Supplement Series](/source/The_Astrophysical_Journal_Supplement_Series)*. **262** (21): 21. [arXiv](/source/ArXiv_(identifier)):[2208.12720](https://arxiv.org/abs/2208.12720). [Bibcode](/source/Bibcode_(identifier)):[2022ApJS..262...21F](https://ui.adsabs.harvard.edu/abs/2022ApJS..262...21F). [doi](/source/Doi_(identifier)):[10.3847/1538-4365/ac7e57](https://doi.org/10.3847%2F1538-4365%2Fac7e57). [S2CID](/source/S2CID_(identifier)) [251864022](https://api.semanticscholar.org/CorpusID:251864022).

1. **[^](#cite_ref-Montes2001_8-0)** Montes, D.; et al. (November 2001). ["Late-type members of young stellar kinematic groups - I. Single stars"](https://doi.org/10.1046%2Fj.1365-8711.2001.04781.x). *[Monthly Notices of the Royal Astronomical Society](/source/Monthly_Notices_of_the_Royal_Astronomical_Society)*. **328** (1): 45–63. [arXiv](/source/ArXiv_(identifier)):[astro-ph/0106537](https://arxiv.org/abs/astro-ph/0106537). [Bibcode](/source/Bibcode_(identifier)):[2001MNRAS.328...45M](https://ui.adsabs.harvard.edu/abs/2001MNRAS.328...45M). [doi](/source/Doi_(identifier)):[10.1046/j.1365-8711.2001.04781.x](https://doi.org/10.1046%2Fj.1365-8711.2001.04781.x). [S2CID](/source/S2CID_(identifier)) [55727428](https://api.semanticscholar.org/CorpusID:55727428).

1. **[^](#cite_ref-Ayres2005_9-0)** Ayres, Thomas R. (January 2005). ["X-Rays from Hybrid Stars"](https://doi.org/10.1086%2F425891). *The Astrophysical Journal*. **618** (1): 493–501. [Bibcode](/source/Bibcode_(identifier)):[2005ApJ...618..493A](https://ui.adsabs.harvard.edu/abs/2005ApJ...618..493A). [doi](/source/Doi_(identifier)):[10.1086/425891](https://doi.org/10.1086%2F425891). [S2CID](/source/S2CID_(identifier)) [123363660](https://api.semanticscholar.org/CorpusID:123363660).

1. **[^](#cite_ref-Bilíková2010_10-0)** Bilíková, Jana; et al. (November 2010). "Hard X-ray Emission Associated with White Dwarfs. III". *The Astronomical Journal*. **140** (5): 1433–1443. [Bibcode](/source/Bibcode_(identifier)):[2010AJ....140.1433B](https://ui.adsabs.harvard.edu/abs/2010AJ....140.1433B). [doi](/source/Doi_(identifier)):[10.1088/0004-6256/140/5/1433](https://doi.org/10.1088%2F0004-6256%2F140%2F5%2F1433).

v t e Constellation of Draco List of stars in Draco Draco in Chinese astronomy Stars Bayer α (Thuban) β (Rastaban) γ (Eltanin) δ (Altais) ε (Tyl) ζ (Aldhibah) η (Athebyne) θ ι (Edasich) κ λ (Giausar) μ (Alrakis) ν (Kuma) ξ (Grumium) ο π ρ σ (Alsafi) τ υ φ χ ψ1 (Dziban) ψ2 ω Flamsteed 4 6 7 (Tianyi) 8 (Taiyi) 10 (i) 15 (A) 18 (g) 19 (h) 26 36 39 (b) 42 (Fafnir) 45 (d) 50 59 64 (e) 68 Variable TW YY AG BY CL CM CX DO EK HP V581 HR 3751 4126 4934 5811 6237 6518 6618 (Alruba) 6817 7137 7783 HD 109246 (Funi) 101364 128717 147379 156279 158259 176693 191939 Other 2MASS 1237+6526 2MASS J17554042+6551277 G 240-72 GD 356 Gliese 420 Gliese 625 Gliese 687 GRW +70 8247 Kepler-4 Kepler-10 Kepler-12 Kepler-90 Kepler-296 KOI-256 LP 71-82 Qatar-1 SDSS J1240+6710 Struve 2398 TOI-1136 WD 1856+534 WISE 1647+5632 WISE 1841+7000 Exoplanets HD 139357 b HD 167042 b ι Dra b (Hypatia) Kepler-4b Kepler-10b c Kepler-12b Kepler-90e f g h i Kepler-296b c d e f TOI-1452 b TrES-2b Star clusters LAMOST 1 Nebulae Cat's Eye Nebula NGC 6742 Galaxies NGC 3147 3197 3735 4121 4125 4221 4236 4319 4331 4332 4363 4441 4513 4545 4589 5678 5777 5866 5866B 5879 5905 5907 5949 5963 5965 5982 5985 6090 6248 6285 6286 6338 6340 6365 6373 6394 6412 6503 6505 6566 6621 6622 6670 6676 6786 6789 Other 1ES 1927+654 1ES 1959+650 3C 249.1 3C 305 3C 319 3C 343 3C 343.1 3C 351 3C 356 3C 371 3C 380 3C 390.3 3C 401 4C +72.26 4C 73.08 8C 1241+735 Abell 2218 BCG Bean galaxy Beaver galaxy BZQ J1727+5510 Draco Dwarf Draco II GB 1508+5714 Goldfish galaxy H1821+643 HFLS3 IERS B1946+708 IRAS F17179+5444 IRAS 18580+6527 J120011.1+680924.8 KKR 25 Markarian 817 Markarian 876 Original TRG PGC 39058 Porphyrion QSO B1823+568 RGZ J172749.5+534647 S5 1803+784 S5 2007+777 SDSS J1715+6008 Tadpole Galaxy UGC 9391 UGC 9405 UGC 11241 UGC 11411 WISEA J145223.62+611707.5 WN B1851+5707 Galaxy clusters Abell 2218 CL1358+62 Draco Supercluster NGC 5866 Group Astronomical events SN 2003jb SN 2005B SN 2016aps Swift J1644+57 Category

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Adapted from the Wikipedia article [HD 81817](https://en.wikipedia.org/wiki/HD_81817) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/HD_81817?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.