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HR 8799

Coordinates:Sky map23h07m28.7150s,+21° 08′ 03.302″
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HR8799

HR 8799 (center) withHR 8799 e(right),HR 8799 d(lower right),HR 8799 c(upper right),HR 8799 b(upper left) fromW. M. Keck Observatory
Observation data
EpochJ2000.0EquinoxJ2000.0
Constellation Pegasus
Right ascension 23h07m28.7157s[1]
Declination +21° 08′ 03.311″[1]
Apparent magnitude(V) 5.964[2]
Characteristics
Spectral type kA5 hF0 mA5 V; λ Boo[3][4]
U−Bcolor index −0.04[5]
B−Vcolor index 0.234[2]
Variable type Gamma Doradus variable[2]
Astrometry
Radial velocity(Rv)−11.5±2[2]km/s
Proper motion(μ)RA:108.284±0.056[1]mas/yr
Dec.:−50.040±0.059[1]mas/yr
Parallax(π)24.4620 ± 0.0455mas[1]
Distance133.3 ± 0.2ly
(40.88 ± 0.08pc)
Absolute magnitude(MV)2.98±0.08[3]
Details
Mass1.43+0.06
−0.07[6]M☉
Radius1.34±0.05[3]R☉
Luminosity (bolometric)4.92±0.41[3]L☉
Surface gravity(logg)4.35±0.05[3]cgs
Temperature7430±75[3]K
Metallicity[Fe/H]−0.52±0.08[7][a]dex
Rotational velocity(vsini)37.5±2[3]km/s
Age30+20
−10[8]Myr
Other designations
V342 Pegasi,BD+20 5278,FK53850,GC32209,HD218396,HIP114189,PPM115157,SAO91022,TYC1718-2350-1.[2]
Database references
SIMBADdata
Exoplanet Archivedata

HR 8799is a roughly 30 million-year-oldmain-sequencestarlocated 133.3light-years(40.9parsecs) away fromEarthin theconstellationofPegasus.It has roughly 1.5 times theSun's mass and 4.9 times its luminosity. It is part of a system that also contains adebris diskand at least fourmassive planets.These planets were the firstexoplanetswhose orbital motion was confirmed bydirect imaging.The star is aGamma Doradus variable:itsluminositychanges because of non-radial pulsations of its surface. The star is also classified as aLambda Boötis star,which means its surface layers are depleted iniron peakelements.It is the only known star which is simultaneously a Gamma Doradus variable, aLambda Boötistype, and aVega-like star (a star withexcess infrared emissioncaused by acircumstellar disk).

Location

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HR 8799 is a star that is visible to the naked eye. It has a magnitude 5.96 and it is located inside the western edge of thegreat square of Pegasusalmost exactly halfway betweenBetaandAlpha Pegasi.The star's name ofHR 8799is its line number in theBright Star Catalogue.

Location of HR 8799

Stellar properties

[edit]
A broadband opticallight curvefor V342 Pegasi (HR 8799), adapted from Sódoret al.(2014)[9]

The star HR 8799 is a member of theLambda Boötis(λBoo) class, a group ofpeculiar starswith an unusual lack of "metals" (elements heavier than hydrogen and helium) in their upper atmosphere. Because of this special status, stars like HR 8799 have a very complex spectral type. The luminosity profile of theBalmer linesin the star's spectrum, as well as the star'seffective temperature,best match the typical properties of anF0 V star.However, the strength of thecalciumII Kabsorption lineand the other metallic lines are more like those of anA5 V star.The star's spectral type is therefore written askA5 hF0 mA5 V;λBoo.[3][4]

Age determination of this star shows some variation based on the method used. Statistically, for stars hosting a debris disk, the luminosity of this star suggests an age of about 20–150 million years. Comparison with stars having similar motion through space gives an age in the range 30–160 million years. Given the star's position on theHertzsprung–Russell diagramof luminosity versus temperature, it has an estimated age in the range of 30–1,128 million years.λBoötis stars like this are generally young, with a mean age of a billion years. More accurately,asteroseismologyalso suggests an age of approximately a billion years.[10]However, this is disputed because it would make the planets become brown dwarfs to fit into the cooling models. Brown dwarfs would not be stable in such a configuration. The best accepted value for an age of HR 8799 is 30 million years, consistent with being a member of theColumba associationco-movinggroup of stars.[11]

Earlier analysis of the star's spectrum reveals that it has a slight overabundance ofcarbonandoxygencompared to the Sun (by approximately 30% and 10% respectively). While some Lambda Boötis stars havesulfurabundances 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 thansodium:for example, the iron abundance is only 28% of the solar iron abundance.[12]Asteroseismicobservations 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.[13]

In 2020, spectral analysis utilizing multiple data sources have detected an inconsistency in prior data and concluded the star carbon and oxygen abundances are the same or slightly higher than solar. The iron abundance was updated to 30+6
−5% of solar value.[7]

Astroseismic analysis using spectroscopic data indicates that the rotational inclination of the star is constrained to be greater than or approximately equal to 40°. This contrasts with the planets' orbital inclinations, which are in roughly the same plane at an angle of about20° ± 10°.Hence, there may be an unexplained misalignment between the rotation of the star and the orbits of its planets.[14]Observation of this star with theChandra X-ray Observatoryindicates that it has a weak level ofmagnetic activity,but the X-ray activity is much higher than that of an A‑type star likeAltair.This suggests that the internal structure of the star more closely resembles that of an F0 star. The temperature of thestellar coronais about 3.0 millionK.[15]

Planetary system

[edit]
The HR 8799 planetary system[8][16][17][18]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(years) 		Eccentricity Inclination Radius
e 7.4±0.6MJ 16.25±0.04 ~45 0.1445±0.0013 25 ± 8° 1.17+0.13
−0.11RJ
d 9.1±0.2MJ 26.67±0.08 ~100 0.1134±0.0011 28° 1.2+0.1
−0RJ
c 7.8±0.5MJ 41.39±0.11 ~190 0.0519±0.0022 28° 1.2+0.1
−0RJ
b 5.7±0.4MJ 71.6±0.2 ~460 0.016±0.001 28° 1.2+0.1
−0.1RJ
Dust disk 135–360[19]AU — —
Orbit diagram of the HR 8799 planetary system

On 13 November 2008, Christian Marois of the National Research Council of Canada'sHerzberg Institute of Astrophysicsand his team announced they had directly observed threeplanetsorbiting the star with theKeckandGeminitelescopes inHawaii,[20][21][22][23]in both cases employingadaptive opticsto make observations in theinfrared.[b]Aprecoveryobservation of the outer 3 planets was later found in infrared images obtained in 1998 by theHubble Space Telescope'sNICMOSinstrument, after a newly developed image-processing technique was applied.[24]Further observations in 2009–2010 revealed the fourth giant planet orbiting inside the first three planets at aprojected separationjust less than 15AU,[8][25]which has been confirmed by multiple studies.[26]

The outer planet orbits are inside a dusty disk like the SolarKuiper 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 forterrestrial planets.[22]There is an additional debris disk just inside the orbit of the innermost planet.[8]

The orbital radii of planetse,d,c,andbare 2–3 times those ofJupiter,Saturn,Uranus,andNeptune's orbits, respectively. Because of theinverse square lawrelatingradiationintensityto distance from the source, comparable radiation intensities are present at distances√4.9≈ 2.2 timesfarther from HR 8799 than from the Sun, the upshot being that corresponding planets in the solar and HR 8799 systems receive similar amounts of stellar radiation.[8]

These objects are near the upper mass limit for classification as planets; if they exceeded 13Jupiter masses,they would be capable ofdeuteriumfusionin their interiors and thus qualify asbrown dwarfsunder the definition of these terms used by theIAU's Working Group on Extrasolar Planets.[27]If the mass estimates are correct, the HR 8799 system is the first multiple-planet extrasolar system to be directly imaged.[21]The orbital motion of the planets is in an anticlockwise direction and was confirmed via multiple observations dating back to 1998.[20]The system is more likely to be stable if the planets e, d, and c are in a 4:2:1 resonance, which would imply that the orbit of the planet d 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 outer three planets are in a 1:2:4orbital resonance(similar to theLaplace resonancebetween Jupiter's inner threeGalilean satellites:Io,Europa,andGanymedeas well as three of the planets in theGliese 876system).[8]However, it is disputed if planet b is in resonance with the other 3 planets. According to dynamical simulations, the HR 8799 planetary system may be even an extrasolar system with multiple resonance 1:2:4:8.[18]The 4 young planets are still glowing red hot from the heat of their formation, and are larger than Jupiter and over time they will cool and shrink to the sizes of 0.8–1.0 Jupiter radii.

The broadband photometry of planets b, c and d has shown that there may be significant clouds in their atmospheres,[25]while the infrared spectroscopy of planets b and c points to non-equilibriumCO/CH4chemistry.[8]Near-infrared observations with theProject 1640integral field spectrograph on the Palomar Observatory have shown that compositions between the four planets vary significantly. This is a surprise since the planets presumably formed in the same way from the same disk and have similar luminosities.[28]

Planet spectra

[edit]
Spectrum of planet around HR 8799. Credit: ESO/M. Janson.
The spectrum is that of a giant exoplanet, orbiting around the bright and very young star HR 8799, about 130 light-years away. This spectrum of the star and the planet was obtained with the NACO adaptive optics instrument onESO'sVery Large Telescope.

A number of studies have used the spectra of HR 8799's planets to determine their chemical compositions and constrain their formation scenarios. The first spectroscopic study of planet b (performed at near-infrared wavelengths) detected strong water absorption and hints of methane absorption.[29]Subsequently, weak methane and carbon monoxide absorption in this planet's atmosphere was also detected, indicating efficient vertical mixing of the atmosphere and a disequilibriumCO/CH4ratio at the photosphere. Compared to models of planetary atmospheres, this first spectrum of planet b is best matched by a model of enhancedmetallicity(about 10 times the metallicity of the Sun), which may support the notion that this planet formed through core-accretion.[30]

The first simultaneous spectra of all four known planets in the HR 8799 system were obtained in 2012 using the Project 1640 instrument at Palomar Observatory. The near-infrared spectra from this instrument confirmed the red colors of all four planets and are best matched by models of planetary atmospheres that include clouds. Though these spectra do not directly correspond to any known astrophysical objects, some of the planet spectra demonstrate similarities with L- and T-typebrown dwarfsand the night-side spectrum of Saturn. The implications of the simultaneous spectra of all four planets obtained with Project 1640 are summarized as follows: Planet b contains ammonia and/or acetylene as well as carbon dioxide, but has little methane; planet c contains ammonia, perhaps some acetylene but neither carbon dioxide nor substantial methane; planet d contains acetylene, methane, and carbon dioxide but ammonia is not definitively detected; planet e contains methane and acetylene but no ammonia or carbon dioxide. The spectrum of planet e is similar to a reddened spectrum of Saturn.[28]

Moderate-resolution near-infrared spectroscopy, obtained with the Keck telescope, definitively detected carbon monoxide and water absorption lines in the atmosphere of planet c. The carbon-to-oxygen ratio, which is thought to be a good indicator of the formation history for giant planets, for planet c was measured to be slightly greater than that of the host star HR 8799. The enhanced carbon-to-oxygen ratio and depleted levels of carbon and oxygen in planet c favor a history in which the planet formed through core accretion.[31]However, it is important to note that conclusions about the formation history of a planet based solely on its composition may be inaccurate if the planet has undergone significant migration, chemical evolution, or core dredging.[clarification needed]Later, in November 2018, researchers confirmed the existence of water and the absence ofmethanein the atmosphere ofHR 8799 cusing high-resolution spectroscopy and near-infrared adaptive optics (NIRSPAO) at the Keck Observatory.[32][33]

The red colors of the planets may be explained by the presence of iron and silicate atmospheric clouds, while their low surface gravities might explain the strong disequilibrium concentrations of carbon monoxide and the lack of strong methane absorption.[31]

Debris disk

[edit]
Spitzer infrared image of HR 8799's debris disk, January 2009. The small dot in the centre is the size of Pluto's orbit.

In January 2009 theSpitzer Space Telescopeobtained images of the debris disk around HR 8799. Three components of the debris disk were distinguished:

  1. Warm dust (T≈ 150 K) orbiting within the innermost planet (e). The inner and outer edges of this belt are close to 4:1 and 2:1 resonances with the planet.[8]
  2. A broad zone of cold dust (T≈ 45 K) with a sharp inner edge orbiting just outside the outermost planet (b). The inner edge of this belt is approximately in 3:2 resonance with said planet, similar toNeptuneand theKuiper belt.[8]
  3. A dramatic halo of small grains originating in the cold dust component.

The halo is unusual and implies a high level of dynamic activity which is likely due to gravitational stirring by the massive planets.[34]The Spitzer team says that collisions are likely occurring among bodies similar to those in the Kuiper Belt and that the three large planets may not yet have settled into their final, stable orbits.[35]

In the photo, the bright, yellow-white portions of the dust cloud come from the outer cold disk. The huge extended dust halo, seen in orange-red, has a diameter of ≈ 2,000AU.The diameter of Pluto's orbit (≈ 80AU) is shown for reference as a dot in the centre.[36]

This disk is so thick that it threatens the young system's stability.[37]

Vortex Coronagraph: Testbed for high-contrast imaging technology

[edit]
Direct image ofexoplanetsaround the star HR 8799 using avortex coronagraphon a 1.5 m portion of theHale Telescope

Up until the year 2010,telescopescould onlydirectly imageexoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger thanJupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation. However, in 2010 a team fromNASAsJet Propulsion Laboratorydemonstrated that avortex coronagraphcould enable small telescopes to directly image planets.[38]They did this by imaging the previously imaged HR 8799 planets using just a 1.5 m portion of theHale Telescope.

NICMOS images

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In 2009, an oldNICMOSimage was processed to show a predicted exoplanet around HR 8799.[39]In 2011, three furtherexoplanetswere rendered viewable in a NICMOS image taken in 1998, using advanced data processing.[39]The image allows the planets' orbits to be better characterised, since they take many decades to orbit their host star.[39]

Search for radio emissions

[edit]

Starting in 2010, astronomers searched for radio emissions from theexoplanetsorbiting HR 8799 using the radio telescope atArecibo Observatory.Despite the large masses, warm temperatures, andbrown dwarf-like luminosities, they failed to detect any emissions at 5 GHz down to a flux density detection threshold of 1.0mJy.[40]

See also

[edit]

Notes

[edit]
  1. ^The star is a member of theLambda Boötisclass ofpeculiar stars,thus the observed abundance may not reflect the abundances of the star as a whole.
  2. ^The planets are young and therefore they are still hot and bright in thenear-infraredpart of the spectrum.

References

[edit]
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[edit]

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