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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
| A Transit Timing Analysis of Nine Rise Light Curves of the Exoplanet System TrES-3
We present nine newly observed transits of TrES-3, taken as part of atransit timing program using the RISE instrument on the LiverpoolTelescope. A Markov-Chain Monte Carlo analysis was used to determine theplanet-star radius ratio and inclination of the system, which were foundto be Rp /R sstarf = 0.1664+0.0011-0.0018 and i = 81.73+0.13-0.04, respectively, consistent with previous results.The central transit times and uncertainties were also calculated, usinga residual-permutation algorithm as an independent check on the errors.A re-analysis of eight previously published TrES-3 light curves wasconducted to determine the transit times and uncertainties usingconsistent techniques. Whilst the transit times were not found to be inagreement with a linear ephemeris, giving χ2 = 35.07 for15 degrees of freedom, we interpret this to be the result of systematicsin the light curves rather than a real transit timing variation. This isbecause the light curves that show the largest deviation from a constantperiod either have relatively little out-of-transit coverage or haveclear systematics. A new ephemeris was calculated using the transittimes and was found to be Tc (0) = 2454632.62610 ±0.00006 HJD and P = 1.3061864 ± 0.0000005 days. The transit timeswere then used to place upper mass limits as a function of the periodratio of a potential perturbing planet, showing that our data aresufficiently sensitive to have probed sub-Earth mass planets in bothinterior and exterior 2:1 resonances, assuming that the additionalplanet is in an initially circular orbit.
| 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.
| The Role of Planet Accretion in Creating the Next Generation of Red Giant Rapid Rotators
Rapid rotation in field red giant stars is a relatively rare butwell-studied phenomenon; here we investigate the potential role ofplanet accretion in spinning up these stars. Using Zahn's theory oftidal friction and stellar evolution models, we compute the decay of aplanet's orbit into its evolving host star and the resulting transfer ofangular momentum into the stellar convective envelope. This experimentassesses the frequency of planet ingestion and rapid rotation on the redgiant branch (RGB) for a sample of 99 known exoplanet host stars. Wefind that the known exoplanets are indeed capable of creating rapidrotators; however, the expected fraction due to planet ingestion is only~ 10% of the total seen in surveys of present-day red giants. Of theplanets ingested, we find that those with smaller initial semimajor axesare more likely to create rapid rotators because these planets areaccreted when the stellar moment of inertia is smallest. We also findthat many planets may be ingested prior to the RGB phase, contrary tothe expectation that accretion would generally occur when the stellarradii expand significantly as giants. Finally, our models suggest thatthe rapid rotation signal from ingested planets is most likely to beseen on the lower RGB, which is also where alternative mechanisms forspin-up, e.g., angular momentum dredged up from the stellar core, do notoperate. Thus, rapid rotators on the lower RGB are the best candidatesto search for definitive evidence of systems that have experiencedplanet accretion.
| The sub-Jupiter mass transiting exoplanet WASP-11b
We report the discovery of a sub-Jupiter mass exoplanet transiting amagnitude {V} = 11.6 host star 1SWASP J030928.54+304024.7. Asimultaneous fit to the transit photometry and radial-velocitymeasurements yield a planet mass Mp = 0.53 ± 0.07{M_J}, radius Rp = 0.91+0.06-0.03 {R_J}and an orbital period of 3.722465^+0.000006-0.000008 days.The host star is an early to mid-K dwarf, with a spectral analysisyielding an effective temperature of 4800 ± 100 K and log g =4.45 ± 0.2. It is amongst the smallest, least massive and lowestluminosity stars known to harbour a transiting exoplanet. WASP-11b isthe third least strongly irradiated transiting exoplanet discovered todate, experiencing an incident flux Fp =1.9×108 erg s-1 cm-2 and havingan equilibrium temperature T_eql = 960 ± 70 K.Photometric measurements and Table 2 are available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/502/395
| 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.
| Homogeneous studies of transiting extrasolar planets - II. Physical properties
I present an homogeneous determination of the physical properties of 14transiting extrasolar planetary systems for which good photometric andspectroscopic data are available. The input quantities for each systemare the results of the light-curve analyses presented in PaperI, andpublished measurements of the stellar velocity amplitude, effectivetemperature and metal abundance. The physical properties are determinedby interpolating within tabulated predictions from stellar theory tofind the optimal match to these input data. Statistical uncertaintiesare found using a perturbation algorithm, which gives a detailed errorbudget for every output quantity. Systematic uncertainties are assessedfor each quantity by comparing the values found using severalindependent sets of stellar models. As a theory-free alternative,physical properties are also calculated using an empirical mass-radiusrelation constructed from high-precision studies of low-mass eclipsingbinary stars.I find that the properties of the planets depend mostly on parametersmeasured from the light and radial velocity curves, and have arelatively minor sensitivity to theoretical predictions. In contrast,the orbital semimajor axes and stellar masses have a strong dependenceon theoretical predictions, and their systematic uncertainties can besubstantially larger than the statistical ones. Using the empiricalmass-radius relation instead, the semimajor axes and stellar masses aresmaller by up to 15 per cent. Thus, our understanding of extrasolarplanets is currently limited by our lack of understanding of low-massstars.Using the properties of all known transiting extrasolar planets, I findthat correlations between their orbital periods, masses and surfacegravities are significant at the 2σ-3σ level. However, theseparation of the known planets into two classes according to theirSafronov number is weaker than previously found, and may not bestatistically significant. Three systems, HAT-P-2, WASP-14 and XO-3,form their own little group of outliers, with eccentric orbits, massiveplanets and stars with masses ~1.3Msolar.The detailed error budgets calculated for each system show where furtherobservations are needed. XO-1 and WASP-1 could do with new transit lightcurves. TrES-2 and WASP-2 would benefit from more precise stellartemperature and abundance measurements. Velocity measurements of theparent stars are vital for determining the planetary masses: TrES-1,XO-1, WASP-1, WASP-2 and the OGLEs need additional data. The homogeneousanalysis presented here is a step towards large-scale statisticalstudies of transiting extrasolar planetary systems, in preparation forthe expected deluge of new detections from CoRoT and Kepler.
| 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.
| WASP-10b: a 3MJ, gas-giant planet transiting a late-type K star
We report the discovery of WASP-10b, a new transiting extrasolar planet(ESP) discovered by the Wide Angle Search for Planets (WASP) Consortiumand confirmed using Nordic Optical Telescope FIbre-fed EchelleSpectrograph and SOPHIE radial velocity data. A 3.09-d period, 29 mmagtransit depth and 2.36 h duration are derived for WASP-10b using WASPand high-precision photometric observations. Simultaneous fitting to thephotometric and radial velocity data using a Markov Chain Monte Carloprocedure leads to a planet radius of 1.28RJ, a mass of2.96MJ and eccentricity of ~0.06. WASP-10b is one of the moremassive transiting ESPs, and we compare its characteristics to thecurrent sample of transiting ESP, where there is currently littleinformation for masses greater than ~2MJ and non-zeroeccentricities. WASP-10's host star, GSC 2752-00114(USNO-B1.01214-0586164) is among the fainter stars in the WASP sample,with V = 12.7 and a spectral type of K5. This result shows promise forfuture late-type dwarf star surveys.
| 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.
| Updated parameters for the transiting exoplanet WASP-3b using RISE, a new fast camera for the Liverpool Telescope
Some of the first results are reported from RISE - a new fast cameramounted on the Liverpool Telescope primarily designed to obtain hightime-resolution light curves of transiting extrasolar planets for thepurpose of transit timing. A full and partial transit of WASP-3 arepresented, and a Markov-Chain Monte Carlo analysis is used to update theparameters from the discovery paper. This results in a planetary radiusof 1.29^+0.05-0.12 RJ and therefore a density of0.82+0.14-0.09~ρ_J, consistent with previousresults. The inclination is 85.06^+0.16-0.15 deg, inagreement (but with a significant improvement in the precision) with thepreviously determined value. Central transit times are found to beconsistent with the ephemeris given in the discovery paper; however, anew ephemeris calculated using the longer baseline results in T_c(0) = 2454 605.55915 ± 0.00023 HJD and P = 1.846835 ± 0.000002days.
| 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.
| HAT-P-7b: An Extremely Hot Massive Planet Transiting a Bright Star in the Kepler Field
We report on the latest discovery of the HATNet project: a very hotgiant planet orbiting a bright (V=10.5) star with a small semimajor axisof a=0.0377+/-0.0005 AU. Ephemeris for the system isP=2.2047299+/-0.0000040 days, midtransit time E=2,453,790.2593+/-0.0010(BJD). Based on the available spectroscopic data on the host star andphotometry of the system, the planet has a mass ofMp=1.78+0.08-0.05 MJ andradius of Rp=1.36+0.20-0.09RJ. The parent star is a slightly evolved F6 star withM*=1.47+0.08-0.05 Msolar,R*=1.84+0.23-0.11 Rsolar,Teff=6350+/-80 K, and metallicity [Fe/H]=+0.26+/-0.08. Therelatively hot and large host star, combined with the close orbit of theplanet, yield a very high planetary irradiance of4.71+1.44-0.05×109 ergcm-2 s-1, which places the planet near the top ofthe pM class of irradiated planets as defined by Fortney et al. If aspredicted by Fortney et al. the planet reradiates its absorbed energybefore distributing it to the night side, the day-side temperatureshould be about 2730+150-100 K. Because the hoststar is quite bright, measurement of the secondary eclipse should befeasible for ground-based telescopes, providing a good opportunity tocompare the predictions of current hot Jupiter atmospheric models withthe observations. Moreover, the host star falls in the field of theupcoming Kepler mission; hence extensive space-borne follow-up,including not only primary transit and secondary eclipse observationsbut also asteroseismology, will be possible.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 NOAO.
| 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.
| WASP-3b: a strongly irradiated transiting gas-giant planet
We report the discovery of WASP-3b, the third transiting exoplanet to bediscovered by the WASP and SOPHIE collaboration. WASP-3b transits itshost star USNO-B1.01256-0285133 every 1.846834 +/- 0.000002 d. Ourhigh-precision radial velocity measurements present a variation withamplitude characteristic of a planetary-mass companion and in phase withthe light curve. Adaptive optics imaging shows no evidence for nearbystellar companions, and line-bisector analysis excludes faint,unresolved binarity and stellar activity as the cause of the radialvelocity variations. We make a preliminary spectroscopic analysis of thehost star and find it to have Teff = 6400 +/- 100K and log g= 4.25 +/- 0.05 which suggests it is most likely an unevolvedmain-sequence star of spectral type F7-8V. Our simultaneous modelling ofthe transit photometry and reflex motion of the host leads us to derivea mass of 1.76+0.08-0.14 MJ and radius1.31+0.07-0.14 RJ for WASP-3b. Theproximity and relative temperature of the host star suggests thatWASP-3b is one of the hottest exoplanets known, and thus has thepotential to place stringent constraints on exoplanet atmosphericmodels.
| WASP-4b: A 12th Magnitude Transiting Hot Jupiter in the Southern Hemisphere
We report the discovery of WASP-4b, a large transiting gas-giant planetwith an orbital period of 1.34 days. This is the first planet to bediscovered by the SuperWASP-South observatory and CORALIE collaborationand the first planet orbiting a star brighter than 16th magnitude to bediscovered in the southern hemisphere. A simultaneous fit tohigh-quality light curves and precision radial velocity measurementsleads to a planetary mass of 1.22+0.09-0.08MJup and a planetary radius of1.42+0.07-0.04 RJup. The host star isUSNO-B1.0 0479-0948995, a G7 V star of visual magnitude 12.5. As aresult of the short orbital period, the predicted surface temperature ofthe planet is 1761 K, making it an ideal candidate for detections of thesecondary eclipse at infrared wavelengths.
| SuperWASP-N extrasolar planet candidates between 18 < RA < 21h
The SuperWASP-I (Wide Angle Search for Planets-I) instrument observed6.7 million stars between 8 and 15mag from La Palma during the 2004May-September season. Our transit-hunting algorithm selected 11626objects from the 184442 stars within the RA (right ascension) range18-21h. We describe our thorough selection procedure whereby catalogueinformation is exploited along with careful study of the SuperWASP datato filter out, as far as possible, transit mimics. We have identified 35candidates which we recommend for follow-up observations.
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