HD 202206

HD 202206 is a yellow dwarf star approximately 148 light-years[1] away in the constellation Capricornus. The star is orbited by a brown dwarf and a planetary companion in a 5:1 resonant configuration.

HD 202206
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Capricornus
Right ascension  21h 14m 57.769s[1]
Declination −20° 47′ 21.15″[1]
Apparent magnitude (V) +8.07[2]
Characteristics
Spectral type G6 V[3]
B−V color index 0.714
Astrometry
Radial velocity (Rv)18.60 ± 0.91[4] km/s
Proper motion (μ) RA: -38.40[1] mas/yr
Dec.: −119.81[1] mas/yr
Parallax (π)22.06 ± 0.82[1] mas
Distance148 ± 5 ly
(45 ± 2 pc)
Absolute magnitude (MV)+4.75
Details
Mass1.15 M☉
Radius1.02 R☉
Luminosity1.07 L☉
Temperature5765 ± 40 K
Metallicity234%
Age5.6 ± 1.2 Gyr
Other designations
BD−21° 5972, HIP 104903, SAO 190163
Database references
SIMBADdata

Distance, age and metallicity

Measurements by the Hipparcos astrometric satellite give a parallax of 22.06 milliarcseconds, which corresponds to a distance of 45 parsecs.[1] It is similar in mass, radius, and luminosity to the Sun, though estimated to be older at around 5.6 Gyr old.[5] It is also more metal-rich than our Sun based on the amount of iron relative to hydrogen.

Planetary system

In 2000, analysis of radial velocity measurements of the star revealed the existence of a brown dwarf companion[6] (designated HD 202206 b) with at least 17 times the mass of Jupiter around the star in an eccentric orbit with a period of around 256 days.[5] This mass exceeds the 13 Jupiter mass limit above which an object can undergo deuterium fusion in its core, which some take to be the dividing line between planets and brown dwarfs, a definition which is supported by the IAU.[7] However, some authors have suggested that the core accretion process (the traditional model for planet formation) can form objects above this limit, up to around 25–30 Jupiter masses.[8] The classification of HD 202206 b as a brown dwarf or "superplanet" is currently unclear.

Even after HD 202206 b was accounted for, the star still showed a drift in the radial velocity measurements, indicating another companion in a longer-period orbit. In 2004 after further observations, the parameters of a companion was announced.[9] This companion is about Jupiter's size, has a planetary mass at least 2.44 times that of Jupiter, and has an eccentric orbit taking around 1384 days to complete. The planet is located in a 5:1 orbital resonance with the inner companion.

The formation history of this planetary system is interesting: depending on the method of formation of the inner companion, the system can either be regarded as consisting of a superplanet and a planet, or alternatively a planet revolving in a circumbinary orbit.

The HD 202206 planetary system
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b >17.4 MJ 0.83 255.87 ± 0.06 0.435 ± 0.001 — —
c >2.44 MJ 2.55 1383.4 ± 18.4 0.267 ± 0.021 — —

See also
  • PSR B1620-26

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. Vizier catalog entry
  2. Høg, E.; et al. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27–L30. Bibcode:2000A&A...355L..27H.
  3. Houk, N.; Smith-Moore, M. (1988). "Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume 4, Declinations -26°.0 to -12°.0". Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. 4. Bibcode:1988mcts.book.....H.
  4. Valenti, Jeff A.; Fischer, Debra A. (2005). "Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs". The Astrophysical Journal Supplement Series. 159: 141. Bibcode:2005ApJS..159..141V. doi:10.1086/430500.
  5. Udry; Mayor, M.; Naef, D.; Pepe, F.; Queloz, D.; Santos, N. C.; Burnet, M. (2002). "The CORALIE survey for southern extra-solar planets VIII. The very low-mass companions of HD 141937, HD 162020, HD 168443, HD 202206: Brown dwarfs or "superplanets"?". Astronomy and Astrophysics. 390 (1): 267–279. arXiv:astro-ph/0202458. Bibcode:2002A&A...390..267U. doi:10.1051/0004-6361:20020685.
  6. "Exoplanets Galore!" (Press release). Garching, Germany: European Southern Observatory. April 15, 2000. Retrieved December 30, 2012.
  7. "Definition of a "Planet"". Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union. Retrieved 2009-07-04.
  8. Mordasini, C.; Alibert; Benz; Naef; et al. (2007). "Giant Planet Formation by Core Accretion". arXiv:0710.5667v1 [astro-ph].
  9. Correia, A.; Udry, S.; Mayor, M.; Laskar, J.; Naef, D.; Pepe, F.; Queloz, D.; Santos, N. C.; et al. (2005). "The CORALIE survey for southern extra-solar planets. XIII. A pair of planets around HD202206 or a circumbinary planet?". Astronomy and Astrophysics. 440 (2): 751–758. arXiv:astro-ph/0411512. Bibcode:2005A&A...440..751C. doi:10.1051/0004-6361:20042376. Archived from the original (abstract) on 2005-10-23.

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