Successful Asteroseismology for a Better Characterization of the Exoplanet HAT-P-7b

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by  M. Oshagh

Successful Asteroseismology for a Better
Characterization of the Exoplanet HAT-P-7b
M.Oshagh,A.Grigahce`ne,O.Benomar,M.-A.Dupret,M.J.P.F.G.Monteiro,
R.ScuflaireandN.C.Santos
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AbstractItiswellknownthatasteroseismologyistheuniquetechniquepermitting

] thestudyoftheinternalstructureofpulsatingstarsusingtheirpulsationalfrequen-
P
cies, which is per se very important.It acquiresan additionalvalue when the star
E
turnsouttobeaplanethost.Inthiscase,theastroseismicstudyoutputmaybeavery
.
h importantinputforthestudyoftheplanetarysystem.Withthisinmind,weusethe
p large time-span of the Kepler public data obtained for the star system HAT-P-7,
-
o firsttoperformanasteroseismicstudyofthepulsatingstarusingTime-Dependent-
r Convection(TDC)models.Secondly,wemakearevisionoftheplanetpropertiesin
t
s thelightoftheastroseismicstudy.
a
[

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v 1 Introduction
0
5
2 Keplerobservationsaretheperfectexampleofsynergybetweentwoseparatefields
0 in Astrophysics. This space-mission initially designed to characterize extra-solar
1. planets is supplying the asteroseismology community with unprecedently precise
0 data.ThehighphotometricaccuracyreachedbyKeplerasrequiredtodetectplanet
3
1
M.Oshagh,A.Grigahce`ne,M.J.P.F.G.MonteiroandN.C.Santos
:
v Centro de Astrof´ısica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
i e-mail:[email protected]
X
M.Oshagh,M.J.P.F.G.MonteiroandN.C.Santos
r
a Departamento de F´ısicaeAstronomia, Faculdade de Cieˆnciasda Universidade doPorto, Porto,
Portugal
O.Benomar
SydneyInstituteforAstronomy(SIfA),SchoolofPhysics,UniversityofSydney,NSW2006,Aus-
tralia
M.-A.DupretandR.Scuflaire
Institutd’AstrophysiqueetdeGe´ophysique,Universite´deLie`ge,Alle´edu6Aouˆt17-B4000Lie`ge,
Belgium
1
2 M.Oshaghetal.
Fig.1 Left:azoom-inofoneofthetransitsobservedinHAT-P-7light-curveobtainedbyKepler.
Originalobserved(blue)andsynthetizedtransit(red)pointsareshown.Seetextformoredetails.
Right:Thesamelight-curveafterremovingthetransit.
transits[5]isactuallyharnessedtoundertakeasteroseismicstudiesofgreatinterest
instellarPhysics.Thisimpliesabettercharacterizationofpulsatingstarsthatharbor
planetarysystemsandconsequentlytighterconstraintsonthepropertiesofthelatter.
Among the 150,000 stars in the Kepler field of view a few of them have been
identifiedasplanethostingstars(TrES-2[8],HAT-P-7[9],andHAT-P-11[3])pre-
viously to the launch of the satellite. Christensen-Dalsgaard et al. 2010 gave an
initialasteroseismicanalysisofKeplerdataforthesestars[4].
HerewegivethepreliminaryresultsofourseismicstudyofHAT-P-7usingthe
Kepler public data (Quarters: Q0-Q2). After a precise and careful cleaning of the
light-curvesfromthe transits, we obtainthe pulsationfrequencies.These frequen-
cies are used as input for a TDC modeling of HAT-P-7 oscillations. In turn, the
obtainedresultsareusedtocharacterizetheorbitingplanet.
2 Keplerdata processing andfrequency analysis
Inthisstudy,wehaveusedpublicshortcadencedatagatheredbyKeplerforHAT-
P-7. The light-curvespansover 144days, correspondingto the quartersQ0-Q2 1.
Anefffectiveastroseismicexploitationofthelightcurveneedsacarefulcleaningof
the59observedtransitswithoutloosingthestructuresinsidethetransitsthatmaybe
causedbythestarpulsations.Mandel&Agol2002[6]presentedamodeltoproduce
synthetictransits in photometriclightcurves.Theirmethodtakes into accountthe
stellar andplanetaryparameters(e.g.star andplanetradii, semimajoraxis, orbital
inclination,stellar limbdarkeningcoefficients)toproducethesynthetictransits.It
isusuallyusedtoconstraintheplanetparameters.
1ThedataareaccessiblethroughtheMultimissionArchiveatSTScIathttp://archive.stsci.edu/.
TDCasteroseismologyforcharacterisationofHAT-P-7b 3
30
Hz)20
m2m/
wer (pp
Po10
0
500 1000 1500 2000 2500 3000
Frequency (mHz)
Fig.2 Left:PowerspectrumofHAT-P-7.Right:E´chellediagramforfrequenciesofdegreeℓ=0-
2inHAT-P-7,wherealargeseparationof59.51m Hzisused.Thefilledcircleswitherrorbarsgive
theobserved frequencies whilethefilledtrianglesgivethefrequencies forthebestTDCmodel.
Thecolorsindicatethesphericaldegreeℓ:black(ℓ=0),blue(ℓ=2)andred(ℓ=1).
For our purpose of cleaning HAT-P-7 light curve, we used this method to pro-
ducesynthetictransitscorrespondingtotheonesobservedinHAT-P-7lightcurve.
We used stellar and planetaryparametersreportedin [11]. We then subtractedthe
obtainedsynthetic transitsfrom the observedones. We endedup with the cleaned
light-curvetobeusedinthefrequencyanalysis.Fig.1givesanexampleofobserved
andprocesseddata.
ThepowerspectrumofHAT-P-7,shownintheleftpanelofFig.2,hasbeenfitted
within a Bayesian framework using a maximum a posteriori approach [2]. Here,
the main formulated assumption is that frequencies vary smoothly as a function
of the radial order n. This allows us to improve the robustness and reliability of
theextractedfrequenciessignificantly,comparedtoaclassicalmaximumlikelihood
approach[1].
Using the obtained frequencies and the atmospheric parameters of HAT-P-7
givenby [9], we search the best modelin a grid of theoreticalmodels.We follow
thesamemethodologyasinGrigahce`neetal.(theseproceedings).
3 Results
The right panel of Fig. 2 gives the e´chelle diagram of HAT-P-7. The diagram is
builtusingalargeseparationof59.51m Hzforfrequenciesofdegreeℓ=0-2.The
observed(filled circles)andtheoreticalfrequencies(filledtriangles)correspondto
three identified spherical degrees illustrated by different colors: black (ℓ=0), blue
(ℓ=2)andred(ℓ=1).Theagreementbetweenmodelandobservationsisverygood.
Thefirst4rowsinTab.1givetheglobalparametersofthestar. Ourresultsare
given in column 4. They are all within the photometric observational boxes. The
obtainedmassandradiusvaluesliebetweentheisochronesfitvaluesof[9]andthe
4 M.Oshaghetal.
Table1 Stellarandplanetaryparameters.
HATNetproject Keplerdata:Q0-Q1 Keplerdata:Q0-Q2
M⋆(M⊙) 1.47a±0.08 1.52b±0.036 1.4150d±0.020
R⋆(R⊙) 1.84a±0.23 1.991b±0.018 1.9276d±0.010
T (K) 6350a±80 – 6423d ±50
eff
Age(Gy) 2.2a±1.0 2.14b±0.26 2.2082d ±0.04
R (R) 1.363a±0.195 1.50c±0.02 1.4588d±0.007
P J
M (M ) 1.776a±0.077 1.82c±0.03 1.709d ±0.024
P J
T (K) 2730a±150 2885c±100 2811d ±22
dayside
a[9]:byfittingisochrones.b[4]:Adiabaticastroseismicstudy.c[11].dThiswork.
adiabaticastroseismicvaluesof[4].TDCmodelsestimationoftheageagreesmore
withtheisochronesfitting[9]thantheadiabaticcalculationsof[4].
Weusethentheseresultsasinputtocharacterizetheplanet.Theresultsareshown
in the bottom part of Tab. 1. We use for that the methods described in [9] for the
estimationofthedaysidetemperatureand[13]fortheotherparameters.Theerror
barsassociatedtoourestimationsaremuchsmallercomparedtotheothers.
In summary, our physically more robust TDC modeling of the oscillations of
HAT-P-7 yields more reliable global parameters for the star. Which gives tighter
constraints on the properties of its orbiting planet. The analysis of the remaining
data obtained by Kepler is expected to give more precise frequency list. We plan
torunmoremodelsinordertobuildadensergrid.Thiswillallowustoexplorea
largerparameterspaceforabettercharacterizationofthestarandhence,itsplanet.
Acknowledgements PartofthisworkwassupportedbyFCT-MCTES-PortugalandFEDER-EC,
throughgrantsPTDC/CTE-AST/098754/2008,SFRH/BPD/41270/2007andSFRH/BPD/47611/2008.
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