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HD 202206

HD 202206 is a yellow dwarf star approximately 151 light-years away in the constellation Capricornus. The star is orbited by two companions with substellar masses in a near-resonant configuration.


Distance, age and metallicity

Measurements by the Hipparcos astrometric satellite give a parallax of 21.58 milliarcseconds, which corresponds to a distance of 46.3 parsecs.[1] It is similar in mass, radius, and luminosity to the Sun, though estimated to be older at around 5.6 Gyr old.[2] 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 substellar companion[3] (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.[2] 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.[4] 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.[5] 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.[6] 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 system
Companion
(in order from star)
Mass Semimajor axis
(AU)
Orbital period
(days)
Eccentricity
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. ^ "HIP 104903". The Hipparcos and Tycho Catalogues. ESA. 1997. http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIP%20104903. Retrieved 26 June 2006.
2. ^ a b Udry et al.; 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. doi:10.1051/0004-6361:20020685. http://www.aanda.org/articles/aa/full/2002/28/aa2416/aa2416.html.
3. ^ "Exoplanets Galore!". European Southern Observatory. 4 May 2000. http://www.eso.org/public/outreach/press-rel/pr-2000/pr-13-00.html. Retrieved 2 August 2009.
4. ^ "Definition of a "Planet"". Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union. http://www.dtm.ciw.edu/users/boss/definition.html. Retrieved 2009-07-04.
5. ^ Mordasini, C. et al. (2007). "Giant Planet Formation by Core Accretion". arΧiv:0710.5667v1 [astro-ph].
6. ^ Correia, A. et al. (2005). "The CORALIE survey for southern extra-solar planets. XIII. A pair of planets around HD202206 or a circumbinary planet?" (abstract). Astronomy and Astrophysics 440: 751–758. doi:10.1051/0004-6361:20042376. http://www.edpsciences.org/articles/aa/abs/2005/35/aa2376-04/aa2376-04.html.


External links

* HD 202206 on SIMBAD
* HD 202206 on Extrasolar Planets Encyclopaedia
* Extrasolar Planet Interactions by Rory Barnes & Richard Greenberg, Lunar and Planetary Lab, University of Arizona

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