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A homogeneous spectroscopic analysis of host stars of transiting planets
Context: The analysis of transiting extra-solar planets provides anenormous amount of information about the formation and evolution ofplanetary systems. A precise knowledge of the host stars is necessary toderive the planetary properties accurately. The properties of the hoststars, especially their chemical composition, are also of interest intheir own right. Aims: Information about planet formation isinferred by, among others, correlations between different parameterssuch as the orbital period and the metallicity of the host stars. Thestellar properties studied should be derived as homogeneously aspossible. The present work provides new, uniformly derived parametersfor 13 host stars of transiting planets. Methods: Effectivetemperature, surface gravity, microturbulence parameter, and ironabundance were derived from spectra of both high signal-to-noise ratioand high resolution by assuming iron excitation and ionizationequilibria. Results: For some stars, the new parameters differfrom previous determinations, which is indicative of changes in theplanetary radii. A systematic offset in the abundance scale with respectto previous assessments is found for the TrES and HAT objects. Ourabundance measurements are remarkably robust in terms of theuncertainties in surface gravities. The iron abundances measured in thepresent work are supplemented by all previous determinations using thesame analysis technique. The distribution of iron abundance then agreeswell with the known metal-rich distribution of planet host stars. Tofacilitate future studies, the spectroscopic results of the current workare supplemented by the findings for other host stars of transitingplanets, for a total dataset of 50 objects.Based on observations made with the Italian Telescopio Nazionale Galileo(TNG) operated on the island of La Palma by the Fundación GalileoGalilei of the INAF (Istituto Nazionale di Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.Based in part on observations made at Observatoire de Haute Provence(CNRS), France.Based on observations made with ESO Telescopes at the La Silla ParanalObservatory under programme ID 080.C-0661.
| Inflating and Deflating Hot Jupiters: Coupled Tidal and Thermal Evolution of Known Transiting Planets
We examine the radius evolution of close in giant planets with a planetevolution model that couples the orbital-tidal and thermal evolution.For 45 transiting systems, we compute a large grid ofcooling/contraction paths forward in time, starting from a large phasespace of initial semimajor axes and eccentricities. Given observationalconstraints at the current time for a given planet (semimajor axis,eccentricity, and system age), we find possible evolutionary paths thatmatch these constraints, and compare the calculated radii toobservations. We find that tidal evolution has two effects. First,planets start their evolution at larger semimajor axis, allowing them tocontract more efficiently at earlier times. Second, tidal heating cansignificantly inflate the radius when the orbit is being circularized,but this effect on the radius is short-lived thereafter. Oftencircularization of the orbit is proceeded by a long period while thesemimajor axis slowly decreases. Some systems with previouslyunexplained large radii that we can reproduce with our coupled model areHAT-P-7, HAT-P-9, WASP-10, and XO-4. This increases the number ofplanets for which we can match the radius from 24 (of 45) to as many as35 for our standard case, but for some of these systems we are requiredto be viewing them at a special time around the era of current radiusinflation. This is a concern for the viability of tidal inflation as ageneral mechanism to explain most inflated radii. Also, large initialeccentricities would have to be common. We also investigate theevolution of models that have a floor on the eccentricity, as may be dueto a perturber. In this scenario, we match the extremely large radius ofWASP-12b. This work may cast some doubt on our ability to accuratelydetermine the interior heavy element enrichment of normal, noninflatedclose in planets, because of our dearth of knowledge about theseplanets' previous orbital-tidal histories. Finally, we find that the endstate of most close in planetary systems is disruption of the planet asit moves ever closer to its parent star.
| Interpreting the yield of transit surveys: are there groups in the known transiting planets population?
Context: Each transiting planet discovered is characterized by 7measurable quantities, that may or may not be linked. This includesthose relative to the planet (mass, radius, orbital period, andequilibrium temperature) and those relative to the star (mass, radius,effective temperature, and metallicity). Correlations between planetmass and period, surface gravity and period, planet radius and startemperature have been previously observed among the 31 known transitinggiant planets. Two classes of planets have been previously identifiedbased on their Safronov number. Aims: We use the CoRoTlux transitsurveys to compare simulated events to the sample of discovered planetsand test the statistical significance of these correlations. Using amodel proved to be able to match the yield of OGLE transit survey, wegenerate a large sample of simulated detections, in which we canstatistically test the different trends observed in the small sample ofknown transiting planets. Methods: We first generate a stellarfield with planetary companions based on radial velocity discoveries,use a planetary evolution model assuming a variable fraction of heavyelements to compute the characteristics of transit events, then apply adetection criterion that includes both statistical and red noisesources. We compare the yield of our simulated survey with the ensembleof 31 well-characterized giant transiting planets, using differentstatistical tools, including a multivariate logistic analysis to assesswhether the simulated distribution matches the known transiting planets. Results: Our results satisfactorily match the distribution ofknown transiting planet characteristics. Our multivariate analysis showsthat our simulated sample and observations are consistent to 76%. Themass vs. period correlation for giant planets first observed with radialvelocity holds with transiting planets. The correlation between surfacegravity and period can be explained as the combined effect of the massvs. period lower limit and by the decreasing transit probability anddetection efficiency for longer periods and higher surface gravity. Ourmodel also naturally explains other trends, like the correlation betweenplanetary radius and stellar effective temperature. Finally, we are alsoable to reproduce the previously observed apparent bimodal distributionof planetary Safronov numbers in 10% of our simulated cases, althoughour model predicts a continuous distribution. This shows that theevidence for the existence of two groups of planets with differentintrinsic properties is not statistically significant.Appendix is only available in electronic form at http://www.aanda.org
| Empirical evidence for tidal evolution in transiting planetary systems
Most transiting planets orbit very close to their parent star, causingstrong tidal forces between the two bodies. Tidal interaction can modifythe dynamics of the system through orbital alignment, circularization,synchronization and orbital decay by exchange of angular moment.Evidence for tidal circularization in close-in giant planet is wellknown. Here, we review the evidence for excess rotation of the parentstars due to the pull of tidal forces towards spin-orbitsynchronization. We find suggestive empirical evidence for such aprocess in the present sample of transiting planetary systems. Thecorresponding angular momentum exchange would imply that some planetshave spiralled towards their star by substantial amounts since thedissipation of the protoplanetary disc. We suggest that this couldquantitatively account for the observed mass-period relation of close-ingas giants. We discuss how this scenario can be further tested and pointout some consequences for theoretical studies of tidal interactions andfor the detection and confirmation of transiting planets from radialvelocity and photometric surveys.
| Evidence for a lost population of close-in exoplanets
We investigate the evaporation history of known transiting exoplanets inorder to consider the origin of observed correlations between mass,surface gravity and orbital period. We show that the survival of theknown planets at their current separations is consistent with a simplemodel of evaporation, but that many of the same planets would not havesurvived closer to their host stars. These putative closer-in systemsrepresent a lost population that could account for the observedcorrelations. We conclude that the relation underlying the correlationsnoted by Mazeh et al. and Southworth et al. is most likely a linearcut-off in the M2/R3 versus a-2 plane,and we show that the distribution of exoplanets in this plane is inclose agreement with the evaporation model.
| Empirical Constraints on Trojan Companions and Orbital Eccentricities in 25 Transiting Exoplanetary Systems
We present a search for Trojan companions to 25 transiting exoplanets.We use the technique of Ford & Gaudi, in which a difference issought between the observed transit time and the transit time that iscalculated by fitting a two-body Keplerian orbit to the radial-velocitydata. This technique is sensitive to the imbalance of mass at the L4/L5points of the planet-star orbit. No companions were detected above2σ confidence. The median 2σ upper limit is 56 M⊕, and the most constraining limit is 2.8 M⊕ for the case of GJ 436. A similar survey usingforthcoming data from the Kepler satellite mission, along with theradial-velocity data that will be needed to confirm transit candidates,will be sensitive to 10-50 M ⊕ Trojan companions in thehabitable zones of their parent stars. As a by-product of this study, wepresent empirical constraints on the eccentricities of the planetaryorbits, including those which have previously been assumed to becircular. The limits on eccentricity are of interest for investigationsof tidal circularization and for bounding possible systematic errors inthe measured planetary radii and the predicted times of secondaryeclipses.
| Falling Transiting Extrasolar Giant Planets
We revisit the tidal stability of extrasolar systems harboring atransiting planet and demonstrate that, independently of any tidalmodel, none, but one (HAT-P-2b) of these planets has a tidal equilibriumstate, which implies ultimately a collision of these objects with theirhost star. Consequently, conventional circularization andsynchronization timescales cannot be defined because the correspondingstates do not represent the endpoint of the tidal evolution. Usingnumerical simulations of the coupled tidal equations for the spin andorbital parameters of each transiting planetary system, we confirm thesepredictions and show that the orbital eccentricity and the stellarobliquity do not follow the usually assumed exponential relaxation butinstead decrease significantly, eventually reaching a zero value onlyduring the final runaway merging of the planet with the star. The onlycharacteristic evolution timescale of all rotational and orbitalparameters is the lifetime of the system, which crucially depends on themagnitude of tidal dissipation within the star. These results imply thatthe nearly circular orbits of transiting planets and the alignmentbetween the stellar spin axis and the planetary orbit are unlikely to bedue to tidal dissipation. Other dissipative mechanisms, for instanceinteractions with the protoplanetary disk, must be invoked to explainthese properties.
| Probing structural and evolutionary properties of exoplanets
We summarise the results of a) a Keck/HIRES Doppler search for planetsorbiting metal-poor dwarfs, and b) a new spectroscopic and photometricanalysis of the transiting planet systems TrES-3 and TrES-4. These twoexperiments have allowed us to address important issues related to thecorrelation between planet frequencies and properties and themetallicity of the hosts. Our results can usefully inform formation,structural, and evolutionary models of gas giant planets.
| Extrasolar Giant Planets and X-Ray Activity
We have carried out a survey of X-ray emission from stars with giantplanets, combining both archival and targeted surveys. Over 230 starshave been currently identified as possessing planets, and roughlyone-third of these have been detected in X-rays. We carry out detailedstatistical analysis on a volume-limited sample of main-sequence starsystems with detected planets, comparing subsamples of stars that haveclose-in planets with stars that have more distant planets. Thisanalysis reveals strong evidence that stars with close-in giant planetsare on average more X-ray active by a factor of ~4 than those withplanets that are more distant. This result persists for various sampleselections. We find that even after accounting for observational samplebias, a significant residual difference still remains. Thisobservational result is consistent with the hypothesis that giantplanets in close proximity to the primary stars influence the stellarmagnetic activity.
| Homogeneous studies of transiting extrasolar planets - I. Light-curve analyses
I present a homogeneous analysis of the transit light curves of 14well-observed transiting extrasolar planets. The light curves aremodelled using JKTEBOP, random errors are measured using Monte Carlosimulations and the effects of correlated noise are included using aresidual-permutation algorithm. The importance of stellar limb darkeningon the light-curve solutions and parameter uncertainties is investigatedusing five different limb darkening laws and including different numbersof coefficients as fitted parameters. The linear limb darkening lawcannot adequately fit the Hubble Space Telescope (HST) photometry ofHD209458, but the other four laws give very similar results to eachother for all transit light curves. In most cases fixing the limbdarkening coefficients at theoretically predicted values does not biasthe results, but does cause the error estimates to be too small. Theavailable theoretical limb darkening coefficients clearly disagree withempirical values measured from the HST light curves of HD209458 limbdarkening must be included as fitted parameters when analysinghigh-quality light curves.In most cases the results of my analysis agree with the values found byother authors, but the uncertainties I find can be significantly larger(by factors of up to 3). Despite these greater uncertainty estimates,the analyses of sets of independent light curves for both HD189733 andHD209458 lead to results which do not agree with each other. Thisdiscrepancy is worst for the ratio of the radii (6.7σ for HD189733and 3.7σ for HD209458), which depends primarily on the depth ofthe transit. It is therefore not due to the analysis method but ispresent in the light curves. These underlying systematic errors cannotbe detected from the reduced data alone unless at least threeindependent light curves are available for an individual planetarysystem.The surface gravities of transiting extrasolar planets are known to becorrelated with their orbital periods. New surface gravity values,calculated from the light-curve results and the stellar spectroscopicorbits, show that this correlation is still present. New high-precisionlight curves are needed for HD149026, OGLE-TR-10, OGLE-TR-56,OGLE-TR-132 and GJ436, and new radial velocity curves for the XO-1,WASP-1, WASP-2 and the OGLE (Optical Gravitational Lensing Experiment)planetary systems.
| Improved Parameters for Extrasolar Transiting Planets
We present refined values for the physical parameters of transitingexoplanets, based on a self-consistent and uniform analysis of transitlight curves and the observable properties of the host stars. Previouslyit has been difficult to interpret the ensemble properties of transitingexoplanets because of the widely different methodologies that have beenapplied in individual cases. Furthermore, previous studies often ignoredan important constraint on the mean stellar density that can be deriveddirectly from the light curve. The main contributions of this work are(1) a critical compilation and error assessment of all reported valuesfor the effective temperature and metallicity of the host stars, (2) theapplication of a consistent methodology and treatment of errors inmodeling the transit light curves, and (3) more accurate estimates ofthe stellar mass and radius based on stellar evolution models,incorporating the photometric constraint on the stellar density. We useour results to revisit some previously proposed patterns andcorrelations within the ensemble. We confirm the mass-period correlationand find evidence for a new pattern within the scatter about thiscorrelation: planets around metal-poor stars are more massive than thosearound metal-rich stars at a given orbital period. Likewise, we confirmthe proposed dichotomy of planets according to their Safronov number,and we find evidence that the systems with small Safronov numbers aremore metal-rich on average. Finally, we confirm the trend that led tothe suggestion that higher metallicity stars harbor planets with agreater heavy-element content.
| HAT-P-4b: A Metal-rich Low-Density Transiting Hot Jupiter
We describe the discovery of HAT-P-4b, a low-density extrasolar planettransiting BD +36 2593, a V=11.2 mag slightly evolved metal-rich late Fstar. The planet's orbital period is 3.056536+/-0.000057 days with amidtransit epoch of 2,454,245.8154 +/- 0.0003 (HJD). Based onhigh-precision photometric and spectroscopic data, and by using transitlight curve modeling, spectrum analysis, and evolutionary models, wederive the following planet parameters: Mp=0.68+/-0.04MJ, Rp=1.27+/-0.05 RJ,ρp=0.41+/-0.06 g cm-3, and a=0.0446+/-0.0012AU. Because of its relatively large radius, together with its assumedhigh metallicity (that of its parent star), this planet adds to thetheoretical challenges of explaining inflated extrasolar planets.Based in part on observations obtained at the W. M. Keck Observatory,which is operated by the University of California and the CaliforniaInstitute of Technology. Keck time has been in part granted by NASA.
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