Evaporation of extrasolar planets

0.68 MB·English

by  A. Lecavelier des Etangs

Bull.Astr.Soc.India(2010)38,137–145
Evaporation of extrasolar planets
2
1
0 A. Lecavelierdes Etangs1
∗
2
1Institutd’astrophysiquedeParis,CNRS/UPMC,98bisbldArago,F-75014Paris,France
n
a
J Received2010November25;accepted2010December02

3
2

Abstract. Thisarticlepresentsareviewontheobservationsandtheoreticalmodeling
]
P oftheevaporationofextrasolarplanets.Theobservationsandtheresultingconstraints
E ontheupperatmosphere(thermosphereandexosphere)ofthe”hot-Jupiters”. arede-
h. scribed. The early observations of the first discovered transiting extrasolar planet,
p HD209458b,allowedthediscoverythatthisplanethasanextendedatmosphereofes-
- capinghydrogen.Subsequentobservationsshowedthepresenceofoxygenandcarbon
o
at very high altitude. These observations give unique constraints on the escape rate
r
t andmechanismintheatmosphereofhot-Jupiters.ThemostrecentLyman-alphaHST
s
a observations of HD189733b and MgII observations of Wasp-12b allow for the first
[
time comparison of the evaporationfrom differentplanets in different environments.

1 Modelstoquantifytheescaperatefromthemeasuredoccultationdepths,andanen-
v ergydiagramtodescribetheevaporationstateofhot-Jupitersarepresented.Usingthis
7 diagram,it isshownthatfew alreadyknownplanetslike GJ876dor CoRot-7bcould
3
beremnantsofformerlygiantplanets.
6
4
.
1 1. Risingtemperature inthe upper atmosphere: thethermosphere
0
2
1
Physical parametersof the upper atmospheresof extrasolar planetup to the exosphere(the so-
:
v calledthermosphere)canbedeterminedusingabsorptionspectroscopyoftransits.Thistechnique
i has been developed in details for HD209458b (Sing et al. 2008a, 2008b; De´sert et al. 2008)
X
where the detailed Temperature-Pressure-altitudeprofile has been estimated from 0.1mbar to
r ∼
a 50mbar. Inparticular,becausetheatmosphericscaleheightisdirectlyrelatedtothetempera-
∼
ture,thetemperaturecanbeeasilydeterminedbymeasurementofvariationoftransitoccultation
depth as a function of wavelength. For instance, when detected the slope of absorption as a
functionofwavelengthduetothe Rayleighscatteringallowsa directdeterminationofthetem-
peratureatthealtitudewhereRayleighscatteringisopticallythick(LecavelierdesEtangsetal.
2008a,2008b).
e-mail:[email protected]
∗
2 A.LecavelierdesEtangs
TheabsorptionprofileofthesodiumhasbeenrecentlysolvedbyVidal-Madjaretal. (2011)
to furtherconstrain the verticalstructure of the HD209458batmosphereoveran altitude range
ofmorethan6500km,correspondingtoapressurerangeof14scaleheightsspanning1millibar
to10 9 barpressures. Theyfoundariseintemperatureabove3mbarpressurelevel,andabove
−
anisothermalatmosphericlayerspanningalmost6scaleheightsinaltitudeasharprisingofthe
temperaturetoabout2500K at 10 9 barshowingthepresenceofathermospherefromwhich
−
∼
thegascanescapeintotheexosphere(Vidal-Madjaretal.2011).
2. HD209458b, an evaporatingplanet
Figure1. Anumericalsimulationofhydrogenatomssensitivetoradiationpressure(0.7timesthestellar
gravitation)aboveanaltitudeof0.5timestheRocheradiuswherethedensityisassumedtobe2 105cm 3
−
×
ispresentedhere.Itcorrespondstoanescapefluxof 1010gs 1.Themeanionizationlifetimeofescaping
−
∼
hydrogenatomsis4hours. Themodelyieldsanatompopulationinacurvedcometaryliketail(seedetails
inVidal-Madjar&Lecavelier2004).
Higher in the atmosphere, transit observations have also revealed evaporation of the hot-
Jupiters closed to their parentstars. For more than ten years, transit observationsallowed dis-
coveries,detection,andcharacterizationofextrasolarobjects(LecavelierdesEtangsetal.1995,
1997, 1999a, 1999b, 2005; Lamers et al. 1997; Nitschelm et al. 2000; He´brard & Lecavelier
des Etangs 2006; Ehrenreich et al. 2006, 2007). In the recent discoveries, the evaporation of
hot-Jupitersopensanewfieldofresearchintheexoplanetfield. Thefieldwasopenwiththeob-
servationaldiscoverythatHD20958bisevaporating(Vidal-Madjaretal.2003,hereafterVM03).
Thisdiscoveryhasbeenchallengedbya recentworkofBenJaffel(2007);butthe apparentdis-
crepancyhasbeensolvedandthe resultobtainedfromthisobservationdataset is strengthened
(Vidal-Madjaretal.2008).
Evaporationofextrasolarplanets 3
In the VM03 program,three transits of HD209458bwere surveyedwith the STIS spectro-
graph on-board HST ( 20 km.s 1 resolution). For each transit, three consecutive HST orbits
−
∼
werescheduledsuchthatthefirstorbitendedbeforethefirstcontacttoserveasareference,the
two followingonesbeingpartlyorentirelywithinthetransit. Anaverage15 4%(1σ)relative
±
intensitydropnearthecenteroftheLyman-αlinewasobservedduringthetransits. Thisislarger
thanexpectedfortheatmosphereofaplanetoccultingonly 1.5%ofthestar.
∼
Becauseofthesmalldistance(8.5R )betweentheplanetandthestar(allowinganintense
∗
heatingoftheplanetanditsclassificationasa“hotJupiter”)theRochelobeisonlyat2.7plane-
taryradii(i.e.3.6R ).Fillingupthislobewithhydrogenatomsgivesamaximumabsorptionof
Jup
10%duringplanetarytransits.Sinceamoreimportantabsorptionwasdetected,hydrogenatoms
∼
coveralargerareacorrespondingtoasphericalobjectof4.3R . ObservedbeyondtheRoche
Jup
limit,thesehydrogenatomsmustbeescapingtheplanet. Independently,thespectralabsorption
width,withblue-shiftedabsorptionupto-130km.s 1alsoshowsthatsomehydrogenatomshave
−
large velocitiesrelative to the planet, exceedingthe escape velocity. This further confirmsthat
hydrogenatomsmustbeescapingtheplanetaryatmosphere.
Theobserved15%intensitydropcouldonlybeexplainedifhydrogenatomsareabletoreach
theRochelobeoftheplanetandthenescape.Toevaluatetheamountofescapingatomsaparticle
simulationwasbuilt,inwhichhydrogenatomsareassumedtobesensitivetothestellargravity
andradiationpressure(LecavelierdesEtangsetal.2008c;seeFig.1).Inthissimulation,escaping
hydrogenatomsexpandinanasymmetriccometaryliketailandareprogressivelyionizedwhen
movingawayfromtheplanet. Atomsintheevaporatingcomaandtailcoveralargeareaofthe
star. Anescapefluxof 1010g.s 1isneededtoexplaintheobservations.Takingintoaccountthe
−
∼
tidalforcesandthetemperatureriseexpectedintheupperatmosphere,theoreticalevaluationsare
ingoodagreementwiththeobservedrate(seereferencesinSect6).
3. Hydrodynamical escape or“Blow-off”
Four transits of HD209458b were then observed, again with the STIS spectrograph on board
HST,butatlowerresolution(Vidal-Madjaretal.2004,hereafterVM04).Thewavelengthdomain
(1180-1710Å) includes Hi as well as Ci , Ci , Ci , Nv , Oi , Si, Siiii, Siv and Feii lines.
During the transits, absorptions are detected in Hi , Oi and Cii (5 2%, 10 3.5% and 6 3%,
± ± ±
respectively). No absorptions are detected for other lines. The 5% mean absorption over the
wholeHiLyman-alphalineisconsistentwiththepreviousdetectionathigherresolution(VM03),
becausethe15%absorptioncoversonly1/3oftheemissionlinewidth(seediscussioninVidal-
Madjaretal.2008).TheabsorptiondepthsinOiandCiishowthatoxygenandcarbonarepresent
in the extendedupperatmosphereof HD209458b. Thesespeciesmustbe carriedoutup tothe
Rochelobeandbeyond,mostlikelyinastate ofhydrodynamicescapeinwhichtheescapecan
bedescribedasaverticalplanetarywindcaringallspeciesincludingtheheavierspecies.
Thispicturehasbeenstrengthenedby thelatest HST UV observationsofHD209458bper-
formedwiththenewCOSspectrographonboardHST.In2009,theAtlantisSpaceShuttleser-
4 A.LecavelierdesEtangs
vicingmissionhasputthenewspectrographCOSwhichis10to20timesmoresensitiveinthe
UV.ObservationsofHD209448btransitshavebeencarriedoutbyFranceetal.(2010)andLin-
skyetal.(2010). Linskyetal.(2010)foundanexospheretransitsignaturewithafluxdecreased
by7.8% 1.3%forthe CIIlineat1334.5Åandmoresurprisinglyby8.2% 1.4%fortheSiIII
± ±
1206.5Åline.Thesehighresolutionobservationsalsoshowfirstdetectionofvelocitystructurein
theexpandingatmosphereofanexoplanet. Linskyetal.(2010)estimatedamass-lossrateinthe
range(8-40) 1010g/s,assumingthatthecarbonabundanceissolar. Thismass-lossrateestimate
×
is consistentwith previousestimates fromhydrogenescape and with theoreticalhydrodynamic
modelsthatincludemetalsintheoutflowinggas.
4. ObservationsofHD189733b
TheobservationoftheHD209458btransitsrevealedthattheatmosphereofthisplanetishydro-
dynamicallyescaping(Sect.2and3). Theseobservationsraisedthequestionoftheevaporation
stateofhot-Jupiters.IstheevaporationspecifictoHD209458borgeneraltohot-Jupiters?What
is the evaporation mechanism, and how does the escape rate depend on the planetary system
characteristics? ThediscoveryofHD189733b(Bouchyetal.2005),aplanettransitingabright
and nearby K0 star (V=7.7), offers the unprecedented opportunity to answer these questions.
Indeed, among the stars harboring transiting planets, HD189733 presents the largest apparent
brightnessinLyman-α,providingcapabilitiestoconstraintheescaperatetohighaccuracy.
An HST program has been developedto observed HI, CII and OI stellar emission lines to
searchforatmosphericabsorptionsduringthetransitsofHD189733b(lecavelierdesEtangset
al.2010).AtransitsignatureintheHiLyman-αlightcurvehasbeendetectedwithatransitdepth
of5.05 0.75%. Thisdepthexceedstheoccultationdepthproducedbytheplanetarydiskalone
±
atthe3.5σlevel. ThisisconfirmedbytheanalysisofthewholespectraredwardoftheLyman-
α line which has enoughphotonsto show a transit signature consistent with the absorption by
theplanetarydiskalone. Therefore,thepresenceofanextendedexosphereofatomichydrogen
around HD 189733bproducing5% absorption of the full unresolvedLyman-α line flux shows
thattheplanetislosinggas. TheLyman-αlightcurvehasbeenfittedbyanumericalsimulation
of escaping hydrogen to constrain the escape rate of atomic hydrogen to be between 109 and
1011g/s, making HD 189733b the second extrasolar planet for which atmospheric evaporation
hasbeendetected(LecavelierdesEtangsetal. 2010).
These observations give new constraints for our understanding of the evaporation of hot-
JupiterbecauseHD189733bhasdifferentcharacteristicsthanHD2095458b. Itis indeeda very
shortperiodplanet(P=2.2days)orbitinganearbylatetypestar(K0V)withbrightchromospheric
emissionlines. Thisnewdetectionofanevaporatingplanetthusprovidesnewinformationonthe
closeconnectionbetweenthestarandplanetfortheseextremeplanetarysystems.
Evaporationofextrasolarplanets 5
5. New casefordetection ofevaporating exosphere : Wasp-12b
Fossati et al. (2010)obtained near-UV transmission spectroscopyof the highly irradiated tran-
siting exoplanetWASP-12b with the COS spectrographon the Hubble Space Telescope. They
detectedenhancedtransitdepthsattributabletoabsorptionbyresonancelinesofmetalsintheex-
osphereofWASP-12blikeabsorptionintheMgIIλ2800Åresonancelinecores.Thisobservation
suggeststhattheplanetissurroundedbyanabsorbingcloudwhichoverfillstheRochelobeand
thereforemustbeescapingtheplanet.
These recent observations show that observation of evaporating exosphere is now feasible
forplanettransitinginfrontof starsfainterthanHD189733borHD209458b,possiblydownto
star as faint as mv 10. More importantly, this definitely shows that evaporation is a common
∼
phenomenonforJupiter-massplanetsorbitingatfewstellarradiifromtheirparentstar.
6. A diagram forthe evaporationstatus ofextrasolarplanets
The observational constraints given in previous sections have been used to developed a large
numberofmodels. Thesemodelsaimatabetterunderstandingoftheobservedescaperateand
evaporationproperties,andsubsequentlydrawingtheconsequenceonotherplanetsandplanetary
systems(Lammeretal. 2003;LecavelierdesEtangsetal. 2004,2007;Baraffeetal. 2004,2005,
2006;Yelle2004,2006;Jaritzetal. 2004;Tianetal. 2005;Garcia-Munoz2007). However,all
themodelingeffortsleadtotheconclusionthatmostoftheEUVandX-rayinputenergybythe
harboringstarisusedbytheatmospheretoescapetheplanetgravitationalpotential.
Therefore, to describe the evaporation status of the extrasolar planets, an energy diagram
as been developedin which the potentialenergy of the planets is plotted versus the energy re-
ceived by the upperatmosphere(Lecavelierdes Etangs2007). Thisdescription allowsa quick
estimate of both the escape rate of the atmospheric gas and the lifetime of a planetagainstthe
evaporationprocess. Intheenergydiagram,thereisanevaporation-forbiddenregioninwhicha
gaseousplanetwouldevaporateinlessthan5billionyears. Withtheirobservedcharacteristics,
allextrasolarplanetsarefoundoutsidethisevaporation-forbiddenregion(Fig.2).
7. Neptune massplanets inthediagram
AplotofthemassdistributionoftheextrasolarplanetsshowsthatNeptune-massandEarth-mass
planetsplayaparticularrole(LecavelierdesEtangs2007).InNovember2010,withradialveloc-
itysearches,forty-six(46!)planetshavebeenfoundwithmassbelow0.08M (25Earth-mass),
Jup
while onlynine(9!) planethasbeenidentifiedwith massin the range0.08M -0.16M (25-
jup jup
50Earth-mass).Thisgapisnotabiasintheradialvelocitysearches,sincemoremassiveplanets
are easier to detect. This revealsthe differentnatureof these Neptune mass planets orbitingat
shortorbitaldistances.Buttheirnatureisstillamatterofdebate(Baraffeetal.2005).Inparticu-
larthequestionarisesiftheycanbetheremnantsofevaporatedmoremassiveplanets(“chthonian
6 A.LecavelierdesEtangs
Figure2. Plot of the potential energy of the extrasolar planets as a function of the mean EUV energy
receivedperbillionofyears,< dE /dt >. Tokeeptheplanetswiththesmallestorbitaldistancesinthe
EUV
leftpartofthediagram,thedirectionoftheabscissaaxisischosenwiththelargestvalueofthemeanenergy
fluxtowardtheleft. Identifiedplanetsareplottedwithsymbolsdependingonthetypeofthecentralstar:
trianglesforFstars,filledcirclesforGstars,diamondsforKstarsandsquaresforMstars;planetsorbiting
classIIIstarsareplottedwithemptycircles.Fromthepositioninthediagram,thetypicallifetimeofagiven
planetcanberapidlyextracted.Ifthemeanenergyflux<dE /dt>isgiveninunitofergperbillionyears,
EUV
andthepotentialenergyisgiveninunitoferg,thesimpleratioofbothquantitiesprovidesthecorresponding
lifetimeinbillionofyears. Inthediagram, lifetimeisochronesarestraightlines. Thelifetimeof5Gyris
plottedwithathickline.
The striking result is the absence of planets in the bottom left region which corresponds to light planets
(small E )atshortorbitaldistances(large< dE /dt >). Aplotofthelifetimelineatt=5Gyr,shows
− ′p EUV
thattherearenoplanetsinthispartofthediagramsimplybecausethisisanevaporation-forbiddenregion.
PlanetsinthisregionwouldreceivemoreEUVenergythanneededtofillthepotentialwelloftheplanet,and
evaporateinlessthan5Gyr,leavingaremainingcore,anevaporationremnant(alsonameda“chthonian”
planet;LecavelierdesEtangsetal.2004).
planets”)asforeseeninLecavelierdesEtangsetal.(2004)? Otherpossibilitiesincludegaseous
Neptune-likeplanets,super-Earthorocean-planets(Kutchner2003,Le´geretal.2004).
We plottedthepositionoftheseNeptunemassplanetsintheenergydiagramwith different
hypothesisontheirdensity(Fig.3). Weusedmeanplanetarydensityofρ =6gcm 3foratypical
p −
density of refractory-richplanetswhich shoulddescribe the chthonianandsuper-Earthplanets.
A lower density on the order of ρ =2gcm 3 can be considered as more plausible for volatile-
p −
richplanetsdescribingtheoceanplanets. Forgas-richplanetswe assumedmuchlowerdensity
ofρ =0.2gcm 3 andρ =0.4gcm 3, describingplanetswhichshouldlookmorelike irradiated
p − p −
Neptune-likeplanets.
Evaporationofextrasolarplanets 7
Figure3.PlotofthepotentialenergyoftheNeptunemassplanetasafunctionoftheEUVfluxforvarious
planets’density. ForGJ581b, GJ436b,HD69830b, 55CnceandGJ876d, andassumingsini = √2 1,
−
lifetimeshorterthan5Gyrareobtained fordensitiesbelow 0.28, 0.55, 0.56, 0.69and3.1gcm 3, respec-
−
tively. Ifsini=1(dottedline),thecritical(minimum)densitiesareincreasedto0.38,0.74,0.78,0.93and
4.2gcm 3,forGJ581b,GJ436b,HD69830b,55Cnce,andGJ876 respectively.
−
GJ876d cannot have the density of an ocean planet and needs to be dense enough to be
locatedabovethe t = 5Gyrlifetimelimit. Thisplanetrequiresadensitylargerthan3.1gcm 3
−
for its atmosphere to survive. GJ876d could be a big rocky planet, like a super-Earth, or a
refractoryremnantofapreviousmoremassiveplanet(anevaporatedoceanplanet?).
In brief, the energy diagram allows us to trace three different categories for the presently
identifiedNeptunemassplanets.Forhalfofthem,theEUVinputenergyseemsnotstrongenough
toaffectsignificantlytheseplanets;wecannotconcludeontheirnature. Foratleastthreeother
planets(GJ436b, 55Cnce andHD69830b), itappearsthattheycannotbe a kindoflow mass
gaseousplanets. Withdensitynecessarilyabove0.5gcm 3 tosurviveevaporation,theseplanets
−
must contain a large fraction of solid/liquid material. Finally, GJ876d must be dense enough,
withadensitylargerthan 3gcm 3,tosurvivethestrongEUVenergyfluxfromitsnearbyparent
−
∼
star. Thisplanetmustcontainalargefractionofmassiveelements(Fig.3).
8. Conclusion
Insummary,theobservationofHILyman-αtransitallowsthedetectionofescapingatmosphere
of HD209458b. The escape rate has been estimated through modeling of the observed tran-
sit light curve, given as estimate around 1010gs 1. The detection of heavy elements has then
−
constrainedtheescapemechanismtobeanhydrodynamicalescape,or“blow-off”.Theevapora-
8 A.LecavelierdesEtangs
tionprocessesareconsistentwithallmeasurementsofthetemperatureprofilesinthehot-Jupiter
upper atmospheres with the presence of temperature rise in thermospheres. Finally, an energy
diagramallowsputtingconstraintsonthedensityoffewHot-Neptunes. Itappearsthatsomeof
thelow-massplanets(masslowerthan 15Earthmass)maybeevaporationremnant,or“chtho-
∼
nianplanets”(LecavelierdesEtangsetal.2004). Presently,themostextremecaseistheplanet
CoRot-7b,orbitinginnomorethan20hoursaroundaK0Vtypestarwithasemi-majoraxisof
0.017AU (about3.6solar radius). With a mass ofabout11 Earth-massanda density ofabout
5gcm 3 (Le´geretal.,2009),thisplanetisverysimilartowhatweexpectfortheremnantcores
−
offormerhot-Jupiterswhoseatmosphereshavebeenevaporated.
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