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Title: WASP-4b Transit Observations With GROND
Authors: Nikolay Nikolov, Thomas Henning, Johannes Koppenhoefer, Monika Lendl, Gracjan Maciejewski, Jochen Greiner

Ground-based simultaneous multiband transit observations allow an accurate system parameters determination and may lead to the detection and characterization of additional bodies via the transit timing variations (TTVs) method. We aimed to (i) characterize the heavily bloated WASP-4b hot Jupiter and its star by measuring system parameters and the dependence of the planetary radius as a function of four (Sloan g', r', i', z') wavelengths and (ii) search for TTVs. We recorded 987 images during three complete transits with the GROND instrument, mounted on the MPG/ESO-2.2m telescope at La Silla Observatory. Assuming a quadratic law for the stellar limb darkening we derive system parameters by fitting a composite transit light curve over all bandpasses simultaneously. To compute uncertainties of the fitted parameters, we employ the Bootstrap Monte Carlo Method. The three central transit times are measured with precision down to 6 s. We find a planetary radius Rp = 1.4130.020 RJup, an orbital inclination i = 88deg57'0.45deg and calculate a new ephemeris, a period P = 1.338231440.00000032days and a reference transit epoch T0 = 2454697.7983110.000046 (BJD). Analysis of the new transit mid-times in combination with previous measurements shows no sign of a TTV signal greater than 20s. We perform simplified numerical simulations to place upper-mass limits of a hypothetical perturber in the WASP-4b system.

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WASP-4
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Title: Starspots and spin-orbit alignment in the WASP-4 exoplanetary system
Authors: Roberto Sanchis-Ojeda, Joshua N. Winn, Matthew J. Holman, Joshua A. Carter, David J. Osip, Cesar I. Fuentes

We present photometry of 4 transits of the exoplanet WASP-4b, each with a precision of approximately 500 ppm and a time sampling of 40-60s. We have used the data to refine the estimates of the system parameters and ephemerides. During two of the transits we observed a short-lived, low-amplitude anomaly that we interpret as the occultation of a starspot by the planet. We also find evidence for a pair of similar anomalies in previously published photometry. The recurrence of these anomalies suggests that the stellar rotation axis is nearly aligned with the orbital axis, or else the star spot would not have remained on the transit chord. By analyzing the timings of the anomalies we find the sky-projected stellar obliquity to be -1_{-12}^{+14} degrees. This result is consistent with (and more constraining than) a recent observation of the Rossiter-McLaughlin effect. It suggests that the planet migration mechanism preserved the initially low obliquity, or else that tidal evolution has realigned the system. Future applications of this method using data from the Corot and Kepler missions will allow spin-orbit alignment to be probed for many other exoplanets.

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RE: WASP-4b
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WASP-4b.kmz
Google Sky file (2kb, kmz)

Position(2000): R.A. 23h34m15.06s Dec. 420341.1

1SWASPJ233415.06420341.1 (USNO-B1.0 0479-0948995)

Teff (K) 5500 150
Mass (solar masses) 0.90 0.07
Radius (solar radius) 1.15 0.28
Spectral Type G7V
Distance (pc) 300 50

-- Edited by Blobrana at 17:49, 2008-01-12

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Title: WASP-4b: a 12th-magnitude transiting hot-Jupiter in the Southern hemisphere
Authors: D.M. Wilson, M. Gillon, C. Hellier, P.F.L. Maxted, F. Pepe, D. Queloz, D.R. Anderson, A. Collier Cameron, B. Smalley, T.A. Lister, S.J. Bentley, A. Blecha, D.J. Christian, B. Enoch, C.A. Haswell, L. Hebb, K. Horne, J. Irwin, Y.C. Joshi, M. Marmier, M. Mayor, N. Parley, D. Pollacco, F. Pont, R. Ryans, D. Segransan, I. Skillen, R.A. Street, S. Udry, R.G. West, P.J. Wheatley

We report the discovery of WASP-4b, a large transiting gas-giant planet with an orbital period of 1.34 days. This is the first planet to be discovered by the SuperWASP-South observatory and CORALIE collaboration and the first planet orbiting a star brighter than 16th magnitude to be discovered in the Southern hemisphere. A simultaneous fit to high-quality lightcurves and precision radial-velocity measurements leads to a planetary mass of 1.27 0.1 MJup and a planetary radius of 1.45 0.08 RJup. The host star is USNO-B1.0 0479-0948995, a G7V star of visual magnitude 12.5. As a result of the short orbital period, the predicted surface temperature of the planet is 1776 K, making it an ideal candidate for detections of the secondary eclipse at infrared wavelengths.

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