HR 8799
From Wikipedia, the free encyclopedia
 HR 8799 (center blob) with HR 8799d (bottom), HR 8799c (upper right), HR 8799b (upper left) |
|
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) |
|
|---|---|
| Constellation | Pegasus |
| Right ascension | 23h 07m 28.7150s[1] |
| Declination | +21° 08′ 03.302″[1] |
| Apparent magnitude (V) | 5.964[1] |
| Characteristics | |
| Spectral type | kA5 hF0 mA5 V; λ Boo[2][3] |
| U-B color index | −0.04[4] |
| B-V color index | 0.234[1] |
| Variable type | Gamma Doradus variable[1] |
| Astrometry | |
| Radial velocity (Rv) | −11.5 ± 2[1] km/s |
| Proper motion (μ) | RA: 107.93 ± 0.60[5] mas/yr Dec.: −49.63 ± 0.46[5] mas/yr |
| Parallax (π) | 25.38 ± 0.70[5] mas |
| Distance | 129 ± 4 ly (39 ± 1 pc) |
| Absolute magnitude (MV) | 2.98 ± 0.08[2] |
| Details | |
| Mass | 1.47 ± 0.30[2] M☉ |
| Radius | 1.34 ± 0.05[2] R☉ |
| Surface gravity (log g) | 4.35 ± 0.05[2] |
| Luminosity (bolometric) | 4.92 ± 0.41[2] L☉ |
| Temperature | 7430 ± 75[2] K |
| Metallicity | [M/H] = −0.47 ± 0.10[2][note 1] |
| Rotational velocity (v sin i) | 37.5 ± 2[2] km/s |
| Age | 60+100−30 million[6] years |
| Other designations | |
| Database references | |
| SIMBAD | data |
| Extrasolar Planets Encyclopaedia |
data |
HR 8799 is a young (~60 million year old) main sequence star located 129 light years (39 parsecs) away from Earth in the constellation of Pegasus, with roughly 1.5 times the Sun's mass and 4.9 times its luminosity. It is part of a system that also contains a debris disk and at least three massive planets (which, along with Fomalhaut b, were the first extrasolar planets whose orbital motion was confirmed via direct imaging). The designation HR 8799 is the star's identifier in the Bright Star Catalogue. The star is a Gamma Doradus variable: its luminosity changes because of non-radial pulsations of its surface. The star is also classified as a Lambda Boötis star, which means its surface layers are depleted in iron peak elements.[2] This may be due to the accretion of metal-poor circumstellar gas.[7] It is the only known star which is simultaneously a Gamma Doradus variable, a Lambda Boötis star, and a Vega-like star (a star with excess infrared emission caused by a circumstellar disk).[8]
Contents |
[edit] Spectral type and metallicity
The star HR 8799 is a member of the Lambda Boötis class of peculiar stars which means it has unusual abundances of metals at its surface. The spectral type is thus complex. The shape of the hydrogen line and the star's effective temperature best match the typical spectrum of an F0 V star. However the strength of the calcium II K line and the metallic lines are more like those of an A5 V star. The star's spectral type is therefore written as kA5 hF0 mA5 V; λ Boo.[2][3]
Detailed analysis of the star's spectrum reveals that it has a slight overabundance of carbon and oxygen compared to the Sun (by approximately 30% and 10% respectively). While some Lambda Boötis stars have sulfur abundances similar to that of the Sun, this is not the case for HR 8799; the sulfur abundance is only around 35% of the solar level. The star is also poor in elements heavier than sodium: for example, the iron abundance is only 28% of the solar iron abundance.[9] Asteroseismic observations of other pulsating Lambda Boötis stars suggest that the peculiar abundance patterns of these stars are confined to the surface only: the bulk composition is likely more normal. This may indicate that the observed element abundances are the result of the accretion of metal-poor gas from the environment around the star.[10]
[edit] Planetary system
On November 13, 2008, Christian Marois of the National Research Council of Canada's Herzberg Institute of Astrophysics and his team announced they had directly observed three planets orbiting the star with the Keck and Gemini telescopes in Hawaii,[8][11][12][13] in both cases employing adaptive optics to make observations in the infrared. The outer planet orbits just inside a dusty disk like the Solar Kuiper belt. It is one of the most massive disks known around any star within 300 light years of Earth, and there is room in the inner system for terrestrial planets.[12]
The orbital radii of planets d, c and b are 2 to 2.5 times those of Saturn, Uranus, and Neptune, respectively. Because of the inverse square law relating radiation intensity to distance from the source, comparable radiation intensities are present at distances 
= 2.2 times farther from HR 8799 than from the sun, meaning that corresponding planets in the solar and HR 8799 systems receive similar amounts of stellar radiation.
These objects are near the upper mass limit for classification as planets; if they exceeded 13 Jupiter masses, they would be capable of deuterium fusion in their interiors and thus qualify as brown dwarfs under the definition of these terms used by the IAU's Working Group on Extrasolar Planets.[14] If the mass estimates are correct, the HR 8799 system is the first multiple-planet extrasolar system to be directly imaged.[11] The orbital motion of the planets is in an anticlockwise direction was confirmed via multiple observations dating back to 2004.[8] While the discovery paper suggests that the orbits are circular and observed nearly face-on, dynamical simulations require the masses of the planets to be much lower than assumed, otherwise the system is unstable on timescales much shorter than the age of the star. The system is more likely to be stable if the inner two planets are in a 2:1 resonance, which would imply the innermost planet's orbit has an eccentricity exceeding 0.04 in order to match the observational constraints. Planetary systems with the best-fit masses from evolutionary models would be stable if the planets are in a 1:2:4 orbital resonance (similar to the Laplace resonance between Jupiter's inner three Galilean satellites: Io, Europa and Ganymede). If confirmed, the HR 8799 planetary system would be the first extrasolar system to be observed with multiple resonances. Such systems would still be stable with planetary masses up to 1.9 times the nominal values.[15]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (years) |
Eccentricity |
|---|---|---|---|---|
| d | 10±3 MJ | ~ 24 | ~ 100 | >0.04[15][note 2] |
| c | 10±3 MJ | ~ 38 | ~ 190 | ? |
| b | 7+4−2 MJ | ~ 68 | ~ 460 | ? |
| Dust disk | 75 AU | |||
[edit] See also
[edit] Notes
- ^ The star is a member of the Lambda Boötis class of peculiar stars, thus the observed abundance may not reflect the abundances of the star as a whole.
- ^ The eccentricity is given for the case that the planet is in a 2:1 resonance with HR 8799 c, as suggested by stability constraints.
[edit] References
- ^ a b c d e f g V* V342 Peg -- Variable Star of gamma Dor type, entry, SIMBAD. Accessed on line November 14, 2008.
- ^ a b c d e f g h i j k Gray, R.O. and Kaye, A.B. (1999). "HR 8799: A Link between γ Doradus Variables and λ Bootis Stars". The Astronomical Journal 118 (6): 2993–2996. doi:. Bibcode: 1999AJ....118.2993G.
- ^ a b Kaye, A.B. et al. (1999). "Gamma Doradus Stars: Defining a New Class of Pulsating Variables". PASP 111 (761): 840–844. doi:. Bibcode: 1999PASP..111..840K.
- ^ HR 8799, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line November 14, 2008.
- ^ a b c van Leeuwen, F. (2007). "HIP 114189". Hipparcos, the New Reduction. http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-out.add=.&-source=I/311/hip2&recno=113804. Retrieved on 2008-10-13.
- ^ a b Schneider, J.. "Notes for star HR 8799". The Extrasolar Planets Encyclopaedia. http://exoplanet.eu/star.php?st=HR+8799. Retrieved on 2008-10-13.
- ^ Gray, R.O. and Corbally, C.J. (2002). "A Spectroscopic Search for λ Bootis and Other Peculiar A-Type Stars in Intermediate-Age Open Clusters". The Astronomical Journal 124 (2): 989–1000. doi:. Bibcode: 2002AJ....124..989G.
- ^ a b c d Marois, Christian; et al. (November 2008). "Direct Imaging of Multiple Planets Orbiting the Star HR 8799". Science 322 (5906): 1348–1352. doi:. http://fr.arxiv.org/abs/0811.2606.
- ^ Kozo, Sadakane (2006). "λ Bootis-Like Abundances in the Vega-Like, γ Doradus Type-Pulsator HD 218396". Publications of the Astronomical Society of Japan 58 (6): 1023–1032. Bibcode: 2006PASJ...58.1023S. http://pasj.asj.or.jp/v58/n6/580611/580611-frame.html.
- ^ Paunzen, E. et al. (1998). "Pulsation in λ Bootis stars". Astronomy and Astrophysics 335: 533–538. Bibcode: 1998A&A...335..533P. http://aa.springer.de/bibs/8335002/2300533/small.htm.
- ^ a b Gemini Observatory (2008-11-13). Gemini Releases Historic Discovery Image of Planetary First Family. Press release. http://www.gemini.edu/node/11151. Retrieved on 2008-11-13.
- ^ a b W. M. Keck Observatory (2008-11-13). Astronomers capture first images of newly-discovered solar system. Press release. http://www.keckobservatory.org/article.php?id=231. Retrieved on 2008-11-13.
- ^ Achenbach, Joel (2008-11-13). "Scientists Publish First Direct Images of Extrasolar Planets", The Washington Post, The Washington Post Company. Retrieved on 13 November 2008.
- ^ "Definition of a "Planet"". Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union. http://www.dtm.ciw.edu/boss/definition.html. Retrieved on 2008-11-16.
- ^ a b Fabrycky, D.C. and Murray-Clay, R.A. (2008). "Stability of the directly imaged multiplanet system HR 8799: resonance and masses". arΧiv: 0812.0011 [astro-ph] Retrieved on 2008-12-02.