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We use the Spitzer Space Telescope to estimate the dayside thermal emission of the exoplanet TrES-3 integrated in the 3.6, 4.5, 5.8, and 8.0 mm bandpasses of the Infrared Array Camera (IRAC) instrument. We observe two secondary eclipses and find relative eclipse depths of 0.00346 +- 0.00035, 0.00372 +- 0.00054, 0.00449 +- 0.00097, and 0.00475 +- 0.00046, respectively, in the four IRAC bandpasses. We combine our results with the earlier K-band measurement of De Mooij et al., and compare them with models of the planetary emission. We find that the planet does not require the presence of an inversion layer in the high atmosphere. This is the first very strongly irradiated planet that does not have a temperature inversion, which indicates that stellar or planetary characteristics other than temperature have an important impact on temperature inversion. De Mooij & Snellen also detected a possible slight offset in the timing of the secondary eclipse in the K band. However, based on our four Spitzer channels, we place a 3s upper limit of |ecos(o)| < 0.0056, where e is the planet's orbital eccentricity and o is the longitude of the periastron. This result strongly indicates that the orbit is circular, as expected from tidal circularization theory.

We present here the results of our observations of TrES-2 using the Infrared Array Camera on Spitzer. We monitored this transiting system during two secondary eclipses, when the planetary emission is blocked by the star. The resulting decrease in flux is 0.127% +- 0.021%, 0.230% +- 0.024%, 0.199% +- 0.054%, and 0.359% +- 0.060% at 3.6 mm, 4.5 mm, 5.8 mm, and 8.0 mm, respectively. We show that three of these flux contrasts are well fit by a blackbody spectrum with T eff = 1500 K, as well as by a more detailed model spectrum of a planetary atmosphere. The observed planet-to-star flux ratios in all four IRAC channels can be explained by models with and without a thermal inversion in the atmosphere of TrES-2, although with different atmospheric chemistry. Based on the assumption of thermochemical equilibrium, the chemical composition of the inversion model seems more plausible, making it a more favorable scenario. TrES-2 also falls in the category of highly irradiated planets which have been theoretically predicted to exhibit thermal inversions. However, more observations at infrared and visible wavelengths would be needed to confirm a thermal inversion in this system. Furthermore, we find that the times of the secondary eclipses are consistent with previously published times of transit and the expectation from a circular orbit. This implies that TrES-2 most likely has a circular orbit, and thus does not obtain additional thermal energy from tidal dissipation of a non-zero orbital eccentricity, a proposed explanation for the large radius of this planet.

We have derived masses and radii for both components in five short-period single-lined eclipsing binary stars discovered by the TrES wide-angle photometric survey for transiting planets. All these systems consist of a visible F-star primary and an unseen M-star secondary (M A [?] 0.8 M , M B [?] 0.45 M ). The spectroscopic orbital solution combined with a high-precision transit light curve for each system gives sufficient information to calculate the density of the primary star and the surface gravity of the secondary. The masses of the primary stars were obtained using stellar evolution models, which requires accurate determinations of metallicities and effective temperatures. In our case, the uncertainty in the metallicity of the primary stars is the most important limiting factor in order to obtain accurate results for the masses and radii of the unseen M-dwarf secondaries. The solutions were compared with results obtained by calculating the radius of the primary stars under the assumption of rotational synchronization with the orbital period and alignment between their spin axis and the axis of the orbit, using the observed broadening of the spectral lines as an indicator of stellar rotation. Four systems show an acceptable match between the two sets of results when their metallicity is allowed to vary around solar values (-0.5 [?] [Fe/H] [?] +0.5), but one system shows a clear mismatch between the two solutions, which may indicate the absence of synchronization or a misalignment between the rotational and orbital axis. When compared to low-mass stellar evolution models, the derived masses and radii of the unseen M dwarfs are inconsistent (three only marginally) with the predicted values, with all of the radii being larger than expected for their masses. These results confirm the discrepancy shown in a previous work between the predicted and observed radii on low-mass binary stars. This work also shows that reliance on the assumption of synchronization to derive the mass and radius of stars in eclipsing single-lined F+M binaries is a useful tool, but may not always be warranted and should be carefully tested against stellar evolution models.

Spectroscopic and eclipsing binary systems offer the best means for determining accurate physical properties of stars, including their masses and radii. The data available for low-mass stars have yielded firm evidence that stellar structure models predict smaller radii and higher effective temperatures than observed, but the number of systems with detailed analyses is still small. In this paper, we present a complete reanalysis of one of such eclipsing systems, CM Dra, composed of two dM4.5 stars. New and existing light curves as well as a radial velocity curve are modeled to measure the physical properties of both components. The masses and radii determined for the components of CM Dra are M 1 = 0.2310 +- 0.0009 M , M 2 = 0.2141 +- 0.0010M , R 1 = 0.2534 +- 0.0019 R , and R 2 = 0.2396 +- 0.0015 R . With relative uncertainties well below the 1% level, these values constitute the most accurate properties to date for fully convective stars. This makes CM Dra a valuable benchmark for testing theoretical models. In comparing our measurements with theory, we confirm the discrepancies previously reported for other low-mass eclipsing binaries. These discrepancies seem likely to be due to the effects of magnetic activity. We find that the orbit of this system is slightly eccentric, and we have made use of eclipse timings spanning three decades to infer the apsidal motion and other related properties.

We report new spectroscopic and photometric observations of the parent stars of the recently discovered transiting planets TrES-3 and TrES-4. A detailed abundance analysis based on high-resolution spectra yields [Fe/H] = -0.19 +- 0.08, T eff = 5650 +- 75 K, and log g = 4.4 +- 0.1 for TrES-3, and [Fe/H] = +0.14 +- 0.09, T eff = 6200 +- 75 K, and log g = 4.0 +- 0.1 for TrES-4. The accuracy of the effective temperatures is supported by a number of independent consistency checks. The spectroscopic orbital solution for TrES-3 is improved with our new radial velocity measurements of that system, as are the light-curve parameters for both systems based on newly acquired photometry for TrES-3 and a reanalysis of existing photometry for TrES-4. We have redetermined the stellar parameters taking advantage of the strong constraint provided by the light curves in the form of the normalized separation a/R [?] (related to the stellar density) in conjunction with our new temperatures and metallicities. The masses and radii we derive are M [?] = 0.928+0.028 -0.048 M , R [?] = 0.829+0.015 -0.022 R , and M [?] = 1.404+0.066 -0.134 M , R [?] = 1.846+0.096 -0.087 R for TrES-3 and TrES-4, respectively. With these revised stellar parameters, we obtain improved values for the planetary masses and radii. We find Mp = 1.910+0.075 -0.080 M Jup, Rp = 1.336+0.031 -0.036 R Jup for TrES-3, and Mp = 0.925 +- 0.082 M Jup, Rp = 1.783+0.093 -0.086 R Jup for TrES-4. We confirm TrES-4 as the planet with the largest radius among the currently known transiting hot Jupiters.

We estimate the strength of the bandpass-integrated thermal emission from the extrasolar planet TrES-4 at 3.6, 4.5, 5.8, and 8.0 m using the Infrared Array Camera on the Spitzer Space Telescope. We find relative eclipse depths of 0.137% +- 0.011%, 0.148% +- 0.016%, 0.261% +- 0.059%, and 0.318% +- 0.044% in these four bandpasses, respectively. We also place a 2s upper limit of 0.37% on the depth of the secondary eclipse in the 16 m IRS peak-up array. These eclipse depths reveal that TrES-4 has an emission spectrum similar to that of HD 209458b, which requires the presence of water emission bands created by a thermal inversion layer high in the atmosphere in order to explain the observed features. TrES-4 receives more radiation from its star than HD 209458b and has a correspondingly higher effective temperature, therefore the presence of a temperature inversion in this planet's atmosphere lends support to the idea that inversions might be correlated with the irradiance received by the planet. We find no evidence for any offset in the timing of the secondary eclipse, and place a 3s upper limit of |ecos(o)| < 0.0058, where e is the planet's orbital eccentricity and o is the argument of pericenter. From this we conclude that tidal heating from ongoing orbital circularization is unlikely to be the explanation for TrES-4's inflated radius.

We describe the discovery of a 0.68+0.52 M eclipsing binary (EB) with an 8.4 day orbital period, found through a systematic search of 10 fields of the Trans-atlantic Exoplanet Survey (TrES). Such long-period low-mass EBs constitute critical test cases for resolving the long-standing discrepancy between the theoretical and observational mass-radius relations at the bottom of the main sequence. It has been suggested that this discrepancy may be related to strong stellar magnetic fields, which are not properly accounted for in current theoretical models. All previously well-characterized low-mass main-sequence EBs have periods of a few days or less, and their components are therefore expected to be rotating rapidly as a result of tidal synchronization, thus generating strong magnetic fields. In contrast, the binary system described here has a period that is more than 3 times longer than previously characterized low-mass main-sequence EBs, and its components rotate relatively slowly. It is therefore expected to have a weaker magnetic field and to better match the assumptions of theoretical stellar models. Our follow-up observations of this EB yield preliminary stellar properties that suggest it is indeed consistent with current models. If further observations confirm a low level of activity in this system, these determinations would provide support for the hypothesis that the mass-radius discrepancy is at least partly due to magnetic activity.

We monitored the Doppler shift of the G0 V star TrES-2 throughout a transit of its giant planet. The anomalous Doppler shift due to stellar rotation (the Rossiter-McLaughlin effect) is discernible in the data, with a signal-to-noise ratio of 2.9, even though the star is a slow rotator. By modeling this effect we find that the planet's trajectory across the face of the star is tilted by -9deg +- 12deg relative to the projected stellar equator. With 98% confidence, the orbit is prograde.

Of the nearby transiting exoplanets that are amenable to detailed study, TrES-2 is both the most massive and the one with the largest impact parameter. We present z-band photometry of three transits of TrES-2. We improve on the estimates of the planetary, stellar, and orbital parameters, in conjunction with the spectroscopic analysis of the host star by Sozzetti and coworkers. We find the planetary radius to be Rp = 1.222 +- 0.038 RJup and the stellar radius to be R[?] = 1.003 +- 0.027 R. The quoted uncertainties include the systematic error due to the uncertainty in the stellar mass (M[?] = 0.980 +- 0.062 M). The timings of the transits have an accuracy of 25 s and are consistent with a uniform period, thus providing a baseline for future observations with the NASA Kepler satellite, whose field of view will include TrES-2.

We report on a spectroscopic determination of the atmospheric parameters and chemical abundance of the parent star of the recently discovered transiting planet TrES-2. A detailed LTE analysis of a set of Fe I and Fe II lines from our Keck spectra yields Teff = 5850 +- 50 K, log g = 4.4 +- 0.1, and [Fe/H] = -0.15 +- 0.10. Several independent checks (e.g., additional spectroscopy, line-depth ratios) confirm the reliability of our spectroscopic Teff estimate. The mass and radius of the star, needed to determine the properties of the planet, are traditionally inferred by comparison with stellar evolution models using Teff and some measure of the stellar luminosity, such as the spectroscopic surface gravity. We apply here a new method in which we use instead of log g the normalized separation a/R[?] (related to the stellar density), directly measurabele from the light curves of transiting planets with much greater precision. With the a/R[?] value from the light-curve analysis of Holman and coworkers and our Teff estimate, we obtain M[?] = 0.980 +- 0.062 M and R[?] = 1.000img1.gif R, and an evolutionary age of 5.1img2.gif Gyr, in good agreement with other constraints (Ca II H and K line cores, lithium abundance, and rotation). The new stellar parameters yield improved values for the planetary mass and radius of Mp = 1.198 +- 0.053 MJ and Rp = 1.220img3.gif RJ, confirming that TrES-2 is the most massive among the currently known nearby (d [?] 300 pc) transiting hot Jupiters. The surface gravity of the planet, log gp = 3.299 +- 0.016, can be derived independently of the knowledge of the stellar parameters (i.e., directly from observations), and with a very high precision rivaling that of the best known double-lined eclipsing binaries.

Driven by the incomplete understanding of the formation of gas giant extrasolar planets and of their mass-radius relationship, several ground-based, wide-field photometric campaigns are searching the skies for new transiting extrasolar gas giants. As part of the Trans-atlantic Exoplanet Survey (TrES), in 2003/2004 we monitored approximately 30,000 stars (9.5 [?] V [?] 15.5) in a 5.7deg x 5.7deg field in Andromeda with three telescopes over 5 months. We identified six candidate transiting planets from the stellar light curves. From subsequent follow-up observations we rejected each of these as an astrophysical false positive, i.e., a stellar system containing an eclipsing binary, whose light curve mimics that of a Jupiter-sized planet transiting a Sunlike star. We discuss here the procedures followed by the TrES team to reject false positives from our list of candidate transiting hot Jupiters. We present these candidates as early examples of the various types of astrophysical false positives found in the TrES campaign, and discuss what we learned from the analysis.

We present precise z-band photometric time series spanning times of transit of the two exoplanets recently discovered by the SuperWASP collaboration. We find planetary radii of 1.44 +- 0.08 and 1.04 +- 0.06 RJ for WASP-1b and WASP-2b, respectively. These error estimates include both random errors in the photometry and also the uncertainty in the stellar masses. Our results are 5 times more precise than the values derived from the discovery data alone. Our measurement of the radius of WASP-2b agrees with previously published models of hot Jupiters that include both a 20 M[?] core of solid material and the effects of stellar insolation. In contrast, we find that the models cannot account for the large size of WASP-1b, even if the planet has no core. Thus, we add WASP-1b to the growing list of hot Jupiters that are larger than expected. This suggests that "inflated" hot Jupiters are more common than previously thought and that any purported explanations involving highly unusual circumstances are disfavored.

We present RIz photometry of four consecutive transits of the newly discovered exoplanet XO-1b. We improve on the estimates of the transit parameters, finding the planetary radius to be RP = 1.184img1.gifRJ, and the stellar radius to be R[?] = 0.928img2.gif R, assuming a stellar mass of M[?] = (1.00 +- 0.03) M. The uncertainties in the planetary and stellar radii are dominated by the uncertainty in the stellar mass. These uncertainties increase by a factor of 2-3 if a more conservative uncertainty of 0.10 M is assumed for the stellar mass. Our estimate of the planetary radius is smaller than that reported by McCullough and coworkers, and the resulting estimate for the mean density of XO-1b is intermediate between that of the low-density planet HD 209458b and the higher density planets TrES-1 and HD 189733b. The timings of the transits have an accuracy ranging from 0.2 to 2.5 minutes and are marginally consistent with a uniform period.

We report g, V, and r photometric time series of HD 149026 spanning predicted times of transit of the Saturn-mass planetary companion, which was recently discovered by Sato and collaborators. We present a joint analysis of our observations and the previously reported photometry and radial velocities of the central star. We refine the estimate of the transit ephemeris to Tc = (2,453,527.87455img1.gif) + (2.87598img2.gif)N (HJD). Assuming that the star has a radius of 1.45 +- 0.10 R and a mass of 1.30 +- 0.10 M, we estimate the planet radius to be (0.726 +- 0.064)RJup, which implies a mean density of 1.07img3.gif g cm-3. This density is significantly greater than predicted for models that include the effects of stellar insolation and in which the planet has only a small core of solid material. Thus, we confirm that this planet likely contains a large core and that the ratio of core mass to total planet mass is more akin to that of Uranus and Neptune than to either Jupiter or Saturn.

We present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet system TrES-1. The data span a predicted time of secondary eclipse, corresponding to the passage of the planet behind the star. In both bands of our observations, we detect a flux decrement with a timing, amplitude, and duration as predicted by published parameters of the system. This signal represents the first direct detection of (i.e., the observation of photons emitted by) a planet orbiting another star. The observed eclipse depths (in units of relative flux) are 0.00066 +- 0.00013 at 4.5 mm and 0.00225 +- 0.00036 at 8.0 mm. These estimates provide the first observational constraints on models of the thermal emission of hot Jupiters. Assuming that the planet emits as a blackbody, we estimate an effective temperature of Tp = 1060 +- 50 K. Under the additional assumptions that the planet is in thermal equilibrium with the radiation from the star and emits isotropically, we find a Bond albedo of A = 0.31 +- 0.14. This would imply that the planet absorbs the majority of stellar radiation incident upon it, a conclusion of significant impact to atmospheric models of these objects. We also compare our data to a previously published model of the planetary thermal emission, which predicts prominent spectral features in our observational bands due to water and carbon monoxide. This model adequately reproduces the observed planet-to-star flux ratio at 8.0 mm; however, it significantly overpredicts the ratio at 4.5 mm. We also present an estimate of the timing of the secondary eclipse, which we use to place a strong constraint on the expression e cos o, where e is the orbital eccentricity and o is the longitude of periastron. The resulting upper limit on e is sufficiently small that we conclude that tidal dissipation is unlikely to provide a significant source of energy interior to the planet.

Wide-field searches for transiting extrasolar giant planets face the difficult challenge of separating true transit events from the numerous false positives caused by isolated or blended eclipsing binary systems. We describe here the investigation of GSC 01944-02289, a very promising candidate for a transiting brown dwarf detected by the Trans-Atlantic Exoplanet Survey (TrES) network. The photometry and radial velocity observations suggested that the candidate was an object of substellar mass in orbit around an F star. However, careful analysis of the spectral line shapes revealed a pattern of variations consistent with the presence of another star whose motion produced the asymmetries observed in the spectral lines of the brightest star. Detailed simulations of blend models composed of an eclipsing binary plus a third star diluting the eclipses were compared with the observed light curve and used to derive the properties of the three components. Using the predicted stellar parameters, we were able to identify a second set of spectral lines corresponding to the primary of the eclipsing binary and derive its spectroscopic orbit. Our photometric and spectroscopic observations are fully consistent with a blend model of a hierarchical triple system composed of an eclipsing binary with G0 V and M3 V components in orbit around a slightly evolved F5 dwarf. The rotational broadening of the spectral lines of the F5 primary (v sin i [?] 34 km s-1) and its brightness relative to the eclipsing binary (~89% of the total light) made the discovery of the true nature of the system particularly difficult. We believe that this investigation will be helpful to other groups pursuing wide-field transit searches as this type of false detection could be more common than true transiting planets and difficult to identify.