HD 81817

HD 81817 is a binary or possibly triple star system[7][4] in the northern circumpolar constellation of Draco. It has an orange hue and is visible to the naked eye with an apparent visual magnitude of 4.28.[2] The system is located at a distance of approximately 990 light years from the Sun based on parallax,[1] and is drifting closer with a radial velocity of −7 km/s.[2] It is a member of the IC 2391 moving group.[8]

HD 81817
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Draco
Right ascension 09h 37m 05.28778s[1]
Declination +81° 19′ 34.9711″[1]
Apparent magnitude (V) 4.28[2]
Characteristics
Spectral type K3III + ? + DA2.8[3]
B−V color index +1.488±0.001[2]
Astrometry
Radial velocity (Rv)−6.98±0.10[2] km/s
Proper motion (μ) RA: −16.62[1] mas/yr
Dec.: −16.41[1] mas/yr
Parallax (π)3.28 ± 0.15 mas[1]
Distance990 ± 50 ly
(300 ± 10 pc)
Absolute magnitude (MV)−3.14[2]
Orbit[4]
CompanionHD 81817 Ab
Period (P)1047.1±8.5 d
Semi-major axis (a)3.3±0.1 AU
Eccentricity (e)0.17±0.07
Periastron epoch (T)2449712±108
Argument of periastron (ω)
(secondary)		320±34°
Details[4]
A
Mass4.3±0.5 M☉
Radius83.8±7.8 R☉
Luminosity1,822.9 L☉
Temperature4,140±55 K
Metallicity [Fe/H]−0.17±0.1 dex
Rotation801 d
Rotational velocity (v sin i)4.7±0.1 km/s
Age150±40 Myr
Ab
Mass≥27.1±2.1, 124+97.39
−91.97 MJup
B
Temperature~20,000[5] K
Other designations
BD+81°302, FK5 910, HD 81817, HIP 47193, HR 3751, SAO 1551, WD 0930+815[6]
Database references
SIMBADdata

The primary component of this system is an aging giant star with a stellar classification of K3III.[3] The star's 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 coming from this system.[5] The star is 32[10] million years old with 4.3[4] times the mass of the Sun. With the supply of hydrogen at its core exhausted, the star has expanded to 109[11] times the Sun's radius. It is radiating 2,885 times the luminosity of the Sun from its photosphere at an effective temperature of 4,043.[11]

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.[7] A 2020 study was unable to confirm or disprove this candidate white dwarf.[4]

In 2020, a substellar companion, probably a brown dwarf, was discovered in orbit around HD 81817 by radial velocity observations. It has a minimum mass of 27.1 MJ, and orbits at 3.3 AU with a period of 1047.1 days and an eccentricity of 0.17. Gaia DR2 astrometry has also detected a companion with a mass of about 124 MJ (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.[4]

References
  1. van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  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.
  3. Holberg, J. B.; et al. (2013). "Where are all the Sirius-like binary systems?". Monthly Notices of the Royal Astronomical Society. 435 (3): 2077. arXiv:1307.8047. Bibcode:2013MNRAS.435.2077H. doi:10.1093/mnras/stt1433.
  4. Bang, Tae-Yang; et al. (June 2020). "Hybrid star HD 81817 accompanied by brown dwarf or substellar companion". Astronomy & Astrophysics. 638: A148. arXiv:2005.09273. Bibcode:2020A&A...638A.148B. doi:10.1051/0004-6361/201936613.
  5. Bilíková, Jana; et al. (November 2010). "Hard X-ray Emission Associated with White Dwarfs. III". The Astronomical Journal. 140 (5): 1433–1443. Bibcode:2010AJ....140.1433B. doi:10.1088/0004-6256/140/5/1433.
  6. "HD 81817". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2020-01-26.
  7. Reimers, D. (July 1984). "Discovery of a white dwarf companion of the "hybrid" K giant HD 81817". Astronomy and Astrophysics. 136: L5–L6. Bibcode:1984A&A...136L...5R.
  8. Montes, D.; et al. (November 2001). "Late-type members of young stellar kinematic groups - I. Single stars". Monthly Notices of the Royal Astronomical Society. 328 (1): 45–63. arXiv:astro-ph/0106537. Bibcode:2001MNRAS.328...45M. doi:10.1046/j.1365-8711.2001.04781.x. S2CID 55727428.
  9. Ayres, Thomas R. (January 2005). "X-Rays from Hybrid Stars". The Astrophysical Journal. 618 (1): 493–501. Bibcode:2005ApJ...618..493A. doi:10.1086/425891.
  10. Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410 (1): 190–200. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. S2CID 118629873.
  11. 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.
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