* Astronomy

Title: The corona and companion of CoRoT-2A. Insights from X-rays and optical spectroscopy
Authors: S. Schröter, S. Czesla, U. Wolter, H.M. Müller, K.F. Huber, J.H.M.M. Schmitt

CoRoT-2 is one of the most unusual planetary systems known to date. Its host star is exceptionally active, showing a pronounced, regular pattern of optical variability caused by magnetic activity. The transiting hot Jupiter, CoRoT-2b, shows one of the largest known radius anomalies. We analyse the properties and activity of CoRoT-2A in the optical and X-ray regime by means of a high-quality UVES spectrum and a 15 ks Chandra exposure both obtained during planetary transits. The UVES data are analysed using various complementary methods of high-resolution stellar spectroscopy. We characterise the photosphere of the host star by deriving accurate stellar parameters such as effective temperature, surface gravity, and abundances. Signatures of stellar activity, Li abundance, and interstellar absorption are investigated to provide constraints on the age and distance of CoRoT-2. Furthermore, our UVES data confirm the presence of a late-type stellar companion to CoRoT-2A that is gravitationally bound to the system. The Chandra data provide a clear detection of coronal X-ray emission from CoRoT-2A, for which we obtain an X-ray luminosity of 1.9e29 erg/s. The potential stellar companion remains undetected in X-rays. Our results indicate that the distance to the CoRoT-2 system is approximately 270 pc, and the most likely age lies between 100 and 300 Ma. Our X-ray observations show that the planet is immersed in an intense field of high-energy radiation. Surprisingly, CoRoT-2A's likely coeval stellar companion, which we find to be of late-K spectral type, remains X-ray dark. Yet, as a potential third body in the system, the companion could account for CoRoT-2b's slightly eccentric orbit.

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Title: Bright optical dayside emission from extrasolar planet CoRoT-2b
Authors: I.A.G. Snellen, E.J.W. de Mooij, A. Burrows

We present our analysis of the red-channel CoRoT data of extrasolar planet CoRoT-2b. A deep secondary eclipse is detected at a level of 1.02±0.20x10^-4, which suggests that all of the planet-signal detected previously in white light by Alonso et al. (2009) originates from the red channel. CoRoT-2b is the coolest exoplanet that has been detected in the optical so far. In contrast to the other planets, its measured brightness temperature of 2170±50 K is significantly higher than its maximum hemisphere-averaged effective day-side temperature. However, it is not expected that a hot Jupiter radiates as a black body, and its thermal spectrum can deviate significantly from a Planck curve. We present models of the planet/star flux ratio as function of wavelength, which are calculated for a T/P profile in radiative and hydrostatic equilibrium, using a self-consistent atmosphere code. These are compared with the CoRoT detection. We estimate that reflected light contributes only at a 10-20% level to the total optical eclipse depth. The models allow for an 'extra absorber' to be inserted at high altitude in the planet's atmosphere. This produces a thermal inversion layer, recently invoked to explain the photometric reversals and flux enhancements seen in some planets in the infrared. In the 0.5-1.5 um wavelength range, the model-spectra of planets with an extra absorber at high altitude, are relatively suppressed in flux compared to those without such absorber. We therefore argue that, in contrast to the other exoplanets detected in the optical so far, CoRoT-2b may not exhibit a significant thermal inversion in its atmosphere, causing its optical brightness temperature to be boosted above its maximum effective day-side temperature.

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Title: The secondary eclipse of the transiting exoplanet CoRoT-2b
Authors: R. Alonso, T. Guillot, T. Mazeh, S. Aigrain, A. Alapini, P. Barge, A. Hatzes, and F. Pont

We present a study of the light curve of the transiting exoplanet CoRoT-2b, aimed at detecting the secondary eclipse and measuring its depth. The data were obtained with the CoRoT satellite during its first run of more than 140 days. After filtering the low frequencies with a pre-whitening technique, we detect a 0.0060 ±0.0020% secondary eclipse centred on the orbital phase 0.494 ±0.006. Assuming a black-body emission of the planet, we estimate a surface brightness temperature of T_{p,CoRoT} =1910^_ K. We provide the planet's equilibrium temperature and re-distribution factors as a function of the unknown amount of reflected light. The upper limit for the geometric albedo is 0.12. The detected secondary is the shallowest ever found.

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Title: Transit mapping of a starspot on CoRoT-2 - Probing a stellar surface by planetary transits
Authors: U. Wolter, J.H.M.M. Schmitt, K.F. Huber, S. Czesla, H.M. Mueller, E.W. Guenther, A.P. Hatzes

We analyse variations in the transit lightcurves of CoRoT-2b, a massive hot Jupiter orbiting a highly active G star. We use one transit lightcurve to eclipse-map a photospheric spot occulted by the planet. In this case study we determine the size and longitude of the eclipsed portion of the starspot and systematically study the corresponding uncertainties. We determine a spot radius between 4.5° and 10.5° on the stellar surface and the spot longitude with a precision of about ± 1 degree. Given the well-known transit geometry of the CoRoT-2 system, this implies a reliable detection of spots on latitudes typically covered by sunspots; also regarding its size the modelled spot is comparable to large spot groups on the Sun. We discuss the future potential of eclipse mapping by planetary transits for the high-resolution analysis of stellar surface features.

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