TW ;Hydrae: evidence of stellar spots instead of a Hot Jupiter

in Astronomy and Astrophysics (2008), 489

Context: TW Hya is a classical T Tauri star that shows significant radial-velocity variations in the optical regime. These variations have been attributed to a 10 M_Jup planet orbiting the star at 0.04 AU ... [more ▼]

Context: TW Hya is a classical T Tauri star that shows significant radial-velocity variations in the optical regime. These variations have been attributed to a 10 M_Jup planet orbiting the star at 0.04 AU. Aims: The aim of this letter is to confirm the presence of the giant planet around TW Hya by (i) testing whether the observed RV variations can be caused by stellar spots and (ii) analyzing new optical and infrared data to detect the signal of the planet companion. Methods: We fitted the RV variations of TW Hya using a cool spot model. In addition, we obtained new high-resolution optical & infrared spectra, together with optical photometry of TW Hya and compared them with previous data. Results: Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54° can reproduce the reported RV variations. The model also predicts a bisector semi-amplitude variation <10 m s[SUP]-1[/SUP], which is less than the errors of the RV measurements discussed in Setiawan et al. (2008, Nature, 451, 38). The analysis of our new optical RV data, with typical errors of 10 m s[SUP]-1[/SUP], shows a larger RV amplitude that varies depending on the correlation mask used. A slight correlation between the RV variation and the bisector is also observed although not at a very significant level. The infrared H-band RV curve is almost flat, showing a small variation (<35 m s[SUP]-1[/SUP]) that is not consistent with the published optical orbit. All these results support the spot scenario rather than the presence of a hot Jupiter. Finally, the photometric data shows a 20% (peak to peak) variability, which is much larger than the 4% variation expected for the modeled cool spot. The fact that the optical data are correlated with the surface of the cross-correlation function points towards hot spots as being responsible for the photometric variability. Conclusions: We conclude that the best explanation for the RV signal observed in TW Hya is the presence of a cool stellar spot and not an orbiting hot Jupiter. Based on observations taken at the VLT (Paranal), under programs 280.C-5064(A) and 075.C-0202(A), and with the CORALIE spectrograph and EulerCAM both at the Euler Swiss telescope (La Silla). [less ▲]

Transiting exoplanets from the CoRoT space mission. IV. CoRoT-Exo-4b: a transiting planet in a 9.2 day synchronous orbit

in Astronomy and Astrophysics (2008), 488

CoRoT, the first space-based transit search, provides ultra-high-precision light curves with continuous time-sampling over periods of up to 5 months. This allows the detection of transiting planets with ... [more ▼]

CoRoT, the first space-based transit search, provides ultra-high-precision light curves with continuous time-sampling over periods of up to 5 months. This allows the detection of transiting planets with relatively long periods, and the simultaneous study of the host star's photometric variability. In this Letter, we report the discovery of the transiting giant planet CoRoT-Exo-4b and use the CoRoT light curve to perform a detailed analysis of the transit and determine the stellar rotation period. The CoRoT light curve was pre-processed to remove outliers and correct for orbital residuals and artefacts due to hot pixels on the detector. After removing stellar variability about each transit, the transit light curve was analysed to determine the transit parameters. A discrete autocorrelation function method was used to derive the rotation period of the star from the out-of-transit light curve. We determine the periods of the planetary orbit and star's rotation of 9.20205 ± 0.00037 and 8.87 ± 1.12 days respectively, which is consistent with this being a synchronised system. We also derive the inclination, i = 90.00_-0.085[SUP]+0.000[/SUP] in degrees, the ratio of the orbital distance to the stellar radius, a/R[SUB]s[/SUB] = 17.36[SUB]-0.25[/SUB][SUP]+0.05[/SUP], and the planet-to-star radius ratio R_p/R_s=0.1047[SUB]-0.0022[/SUB][SUP]+0.0041[/SUP]. We discuss briefly the coincidence between the orbital period of the planet and the stellar rotation period and its possible implications for the system's migration and star-planet interaction history. The CoRoT space mission, launched on December 27th 2006, has been developed and is operated by CNES, with the contribution of Austria, Belgium, Brazil, ESA, Germany, and Spain. The first CoRoT data will be available to the public in February 2009 from the CoRoT archive: http://idoc-corot.ias.u-psud.fr/ Figures 1, 4 and 5 are only available in electronic form at http://www.aanda.org [less ▲]

A transiting planet among 23 new near-threshold candidates from the OGLE survey - OGLE-TR-182

in Astronomy and Astrophysics (2008), 487

By re-processing the data of the second season of the OGLE survey for planetary transits and adding new mesurements on the same fields gathered in subsequent years with the OGLE telescope, we have ... [more ▼]

By re-processing the data of the second season of the OGLE survey for planetary transits and adding new mesurements on the same fields gathered in subsequent years with the OGLE telescope, we have identified 23 new transit candidates, recorded as OGLE-TR-178 to OGLE-TR-200. We studied the nature of these objects with the FLAMES/UVES multi-fiber spectrograph on the VLT. One of the candidates, OGLE-TR-182, was confirmed as a transiting gas giant planet on a 4-day orbit. We characterised it with further observations using the FORS1 camera and UVES spectrograph on the VLT. OGLE-TR-182b is a typical ``hot Jupiter'' with an orbital period of 3.98 days, a mass of 1.01 ± 0.15~M_Jup and a radius of 1.13[SUP]+0.24[/SUP][SUB]-0.08[/SUB]~R_Jup. Confirming this transiting planet required a large investment in telescope time with the best instruments available, and we comment on the difficulty of the confirmation process for transiting planets in the OGLE survey. We delineate the zone were confirmation is difficult or impossible, and discuss the implications for the CoRoT space mission in its quest for transiting telluric planets. Based on observations made with the FORS1 camera and the FLAMES/UVES spectrograph at the VLT, ESO, Chile (programmes 076.C-0706 and 177.C-0666) and 1.3-m Warsaw Telescope at Las Campanas Observatory, Chile. [less ▲]

Pushing the precision limit of ground-based eclipse photometry

Report (2008)

Until recently, it was considered by many that ground-based photometry could not reach the high cadence sub-mmag regime because of the presence of the atmosphere. Indeed, high frequency atmospheric noises ... [more ▼]

Until recently, it was considered by many that ground-based photometry could not reach the high cadence sub-mmag regime because of the presence of the atmosphere. Indeed, high frequency atmospheric noises (mainly scintillation) limit the precision that high SNR photometry can reach within small time bins. If one is ready to damage the sampling of his photometric time-series, binning the data (or using longer exposures) allows to get better errors, but the obtained precision will be finally limited by low frequency noises. To observe several times the same planetary eclipse and to fold the photometry with the orbital period is thus generally considered as the only option to get very well sampled and precise eclipse light curve from the ground. Nevertheless, we show here that reaching the sub-mmag sub-min regime for one eclipse is possible with a ground-based instrument. This has important implications for transiting planets characterization, secondary eclipses measurement and small planets detection from the ground. [less ▲]

Double-blind test program for astrometric planet detection with Gaia

in Astronomy and Astrophysics (2008), 482

Aims: The scope of this paper is twofold. First, it describes the simulation scenarios and the results of a large-scale, double-blind test campaign carried out to estimate the potential of Gaia for ... [more ▼]

Aims: The scope of this paper is twofold. First, it describes the simulation scenarios and the results of a large-scale, double-blind test campaign carried out to estimate the potential of Gaia for detecting and measuring planetary systems. The identified capabilities are then put in context by highlighting the unique contribution that the Gaia exoplanet discoveries will be able to bring to the science of extrasolar planets in the next decade. <BR />Methods: We use detailed simulations of the Gaia observations of synthetic planetary systems and develop and utilize independent software codes in double-blind mode to analyze the data, including statistical tools for planet detection and different algorithms for single and multiple Keplerian orbit fitting that use no a priori knowledge of the true orbital parameters of the systems. <BR />Results: 1) Planets with astrometric signatures α≃ 3 times the assumed single-measurement error σ_ψ and period P≤ 5 yr can be detected reliably and consistently, with a very small number of false positives. 2) At twice the detection limit, uncertainties in orbital parameters and masses are typically 15-20%. 3) Over 70% of two-planet systems with well-separated periods in the range 0.2≤ P≤ 9 yr, astrometric signal-to-noise ratio 2≤α/σ_ψ≤ 50, and eccentricity e≤ 0.6 are correctly identified. 4) Favorable orbital configurations (both planets with P≤ 4 yr and α/σ_ψ≥ 10, redundancy over a factor of 2 in the number of observations) have orbital elements measured to better than 10% accuracy > 90% of the time, and the value of the mutual inclination angle i_rel determined with uncertainties ≤ 10°. 5) Finally, nominal uncertainties obtained from the fitting procedures are a good estimate of the actual errors in the orbit reconstruction. Extrapolating from the present-day statistical properties of the exoplanet sample, the results imply that a Gaia with σ_ψ = 8 μas, in its unbiased and complete magnitude-limited census of planetary systems, will discover and measure several thousands of giant planets out to 3-4 AUs from stars within 200 pc, and will characterize hundreds of multiple-planet systems, including meaningful coplanarity tests. Finally, we put Gaia's planet discovery potential into context, identifying several areas of planetary-system science (statistical properties and correlations, comparisons with predictions from theoretical models of formation and evolution, interpretation of direct detections) in which Gaia can be expected, on the basis of our results, to have a relevant impact, when combined with data coming from other ongoing and future planet search programs. [less ▲]

Transiting exoplanets from the CoRoT space mission. II. CoRoT-Exo-2b: a transiting planet around an active G star

in Astronomy and Astrophysics (2008), 482

Context: The CoRoT mission, a pioneer in exoplanet searches from space, has completed its first 150 days of continuous observations of ~12 000 stars in the galactic plane. An analysis of the raw data ... [more ▼]

Context: The CoRoT mission, a pioneer in exoplanet searches from space, has completed its first 150 days of continuous observations of ~12 000 stars in the galactic plane. An analysis of the raw data identifies the most promising candidates and triggers the ground-based follow-up. Aims: We report on the discovery of the transiting planet CoRoT-Exo-2b, with a period of 1.743 days, and characterize its main parameters. Methods: We filter the CoRoT raw light curve of cosmic impacts, orbital residuals, and low frequency signals from the star. The folded light curve of 78 transits is fitted to a model to obtain the main parameters. Radial velocity data obtained with the SOPHIE, CORALIE and HARPS spectrographs are combined to characterize the system. The 2.5 min binned phase-folded light curve is affected by the effect of sucessive occultations of stellar active regions by the planet, and the dispersion in the out of transit part reaches a level of 1.09×10[SUP]-4[/SUP] in flux units. Results: We derive a radius for the planet of 1.465 ± 0.029 R_Jup and a mass of 3.31 ± 0.16 M_Jup, corresponding to a density of 1.31 ± 0.04 g/cm^3. The large radius of CoRoT-Exo-2b cannot be explained by current models of evolution of irradiated planets. Based on observations obtained with CoRoT, a space project operated by the French Space Agency, CNES, with participation of the Science Programme of ESA, ESTEC/RSSD, Austria, Belgium, Brazil, Germany and Spain; and on observations made with SOPHIE spectrograph at Observatoire de Haute Provence, France (PNP.07 A.MOUT), CORALIE, and HARPS spectrograph at ESO La Silla Observatroy (079.C-0127/F)). Table 2 is only available in electronic form at http://www.aanda.org [less ▲]

OGLE-TR-211 - a new transiting inflated hot Jupiter from the OGLE survey and ESO LP666 spectroscopic follow-up program

in Astronomy and Astrophysics (2008), 482

We present results of the photometric campaign for planetary and low-luminosity object transits conducted by the OGLE survey in the 2005 season (Campaign #5). About twenty of the most promising candidates ... [more ▼]

We present results of the photometric campaign for planetary and low-luminosity object transits conducted by the OGLE survey in the 2005 season (Campaign #5). About twenty of the most promising candidates discovered in these data were subsequently verified spectroscopically with the VLT/FLAMES spectrograph. One of the candidates, OGLE-TR-211, reveals clear changes of radial velocity with a small amplitude of 82 m/s, varying in phase with photometric transit ephemeris. Further analysis confirms the planetary nature of this system. Follow-up precise photometry of OGLE-TR-211 with VLT/FORS, together with radial velocity spectroscopy, supplemented with high-resolution, high S/N VLT/UVES spectra allowed us to derive parameters of the planet and host star. OGLE-TR-211b is a hot Jupiter orbiting an F7-8 spectral type dwarf star with a period of 3.68 days. The mass of the planet is equal to 1.03±0.20 M_Jup, while its radius 1.36[SUP]+0.18[/SUP][SUB]-0.09[/SUB] R_Jup. The radius is about 20% larger than the typical radius of hot Jupiters of similar mass. OGLE-TR-211b is, then, another example of inflated hot Jupiters - a small group of seven exoplanets with large radii and unusually low densities - objects that are a challenge to the current models of exoplanets. Based on observations made with the FORS1 camera and the FLAMES/UVES spectrograph at the VLT, ESO, Chile (program 07.C-0706, 076.C-0122, and 177.C-0666) and 1.3-m Warsaw Telescope at Las Campanas Observatory, Chile. [less ▲]

Characterization of the hot Neptune GJ 436 b with Spitzer and ground-based observations

in Astronomy and Astrophysics (2007), 475

We present Spitzer Space Telescope infrared photometry of a secondary eclipse of the hot Neptune GJ 436 b. The observations were obtained using the 8-mum band of the InfraRed Array Camera (IRAC). The data ... [more ▼]

We present Spitzer Space Telescope infrared photometry of a secondary eclipse of the hot Neptune GJ 436 b. The observations were obtained using the 8-mum band of the InfraRed Array Camera (IRAC). The data spanning the predicted time of secondary eclipse show a clear flux decrement with the expected shape and duration. The observed eclipse depth of 0.58 mmag allows us to estimate a blackbody brightness temperature of T[SUB]p[/SUB] = 717 ± 35 K at 8 mum. We compare this infrared flux measurement to a model of the planetary thermal emission, and show that this model reproduces properly the observed flux decrement. The timing of the secondary eclipse confirms the non-zero orbital eccentricity of the planet, while also increasing its precision (e = 0.14 ± 0.01). Additional new spectroscopic and photometric observations allow us to estimate the rotational period of the star and to assess the potential presence of another planet. Our final secondary eclipse, photometric and Ca II H+K index time series are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/475/1125 [less ▲]

Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b

in Astronomy and Astrophysics (2007), 471

We present Spitzer Space Telescope infrared photometry of a primary transit of the hot Neptune GJ 436b. The observations were obtained using the 8 mum band of the InfraRed Array Camera (IRAC). The high ... [more ▼]

We present Spitzer Space Telescope infrared photometry of a primary transit of the hot Neptune GJ 436b. The observations were obtained using the 8 mum band of the InfraRed Array Camera (IRAC). The high accuracy of the transit data and the weak limb-darkening in the 8 mum IRAC band allow us to derive (assuming M = 0.44 ± 0.04 M_o for the primary) a precise value for the planetary radius (4.19[SUP]+0.21[/SUP][SUB]-0.16[/SUB] R_â ), the stellar radius (0.463[SUP]+0.022[/SUP][SUB]-0.017[/SUB] R_o), the orbital inclination (85.90°[SUP]+0.19°[/SUP][SUB]-0.18°[/SUB]) and transit timing (2454280.78186[SUP]+0.00015[/SUP][SUB]-0.00008[/SUB] HJD). Assuming current planet models, an internal structure similar to that of Neptune with a small H/He envelope is necessary to account for the measured radius of GJ 436b. The photometric data are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/471/L51 [less ▲]

First Results from SuperWASP

in Transiting Extrapolar Planets Workshop (2007, July 01)

We present a summary of the first results from the SuperWASP survey, including the detection of two new transiting exoplanets. We summarize our candidate selection procedure and the process by which we ... [more ▼]

We present a summary of the first results from the SuperWASP survey, including the detection of two new transiting exoplanets. We summarize our candidate selection procedure and the process by which we eliminated many false positives prior to radial velocity observations carried out with the Sophie spectrograph at the Observatoire de Haute-Provence. These data confirmed the discovery of two new transiting hot Jupiters, while rejecting 23 other targets. The two confirmed planets, WASP-1b & WASP-2b, respectively orbit F7V and K1V host stars with periods of 2.52 days and 2.15 days. The mass of WASP-1b is constrained to the range 0.80-0.98 M_{Jup} and the planet appears to be `bloated' with a radius of at least 1.33 R_{Jup}. WASP-2b has a mass between 0.81-0.95 M_{Jup} and a radius in the range 0.65-1.26 R_{Jup}. [less ▲]