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Title: Radiative Hydrodynamic Simulations of HD209458b: Temporal Variability
Authors: Ian Dobbs-Dixon, Andrew Cumming, D.N.C Lin

We present a new approach for simulating the atmospheric dynamics of the close-in giant planet HD209458b that allows for the decoupling of radiative and thermal energies, direct stellar heating of the interior, and the solution of the full 3D Navier Stokes equations. Simulations reveal two distinct temperature inversions (increasing temperature with decreasing pressure) at the sub-stellar point due to the combined effects of opacity and dynamical flow structure and exhibit instabilities leading to changing velocities and temperatures on the nightside for a range of viscosities. Imposed on the quasi-static background, temperature variations of up to 15% are seen near the terminators and the location of the coldest spot is seen to vary by more than 20 degrees, occasionally appearing west of the anti-solar point. Our new approach introduces four major improvements to our previous methods including simultaneously solving both the thermal energy and radiative equations in both the optical and infrared, incorporating updated opacities, including a more accurate treatment of stellar energy deposition that incorporates the opacity relevant for higher energy stellar photons, and the addition of explicit turbulent viscosity.

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HD 209458 b is an extrasolar planet that orbits the Solar analogue star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapour.  The radius of the planet's orbit is 7 million kilometres, about 0.047 astronomical units, or one eighth the radius of Mercury's orbit.
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HD 209458b
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Title: Water in HD 209458b's atmosphere from 3.6 - 8 microns IRAC photometric observations in primary transit
Authors: J.P. Beaulieu, D.M. Kipping, V. Batista, G. Tinetti, I. Ribas, S. Carey, J. A. Noriega-Crespo, C. A. Griffith, G. Campanella, S. Dong, J. Tennyson, R.J. Barber, P. Deroo, S.J. Fossey, D. Liang, M. R. Swain, Y. Yung, N. Allard

The hot Jupiter HD 209458b was observed during primary transit at 3.6, 4.5, 5.8 and 8.0 microns using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We detail here the procedures we adopted to correct for the systematic trends present in the IRAC data. The light curves were fitted including limb darkening effects and fitted using Markov Chain Monte Carlo and prayer-bead Monte Carlo techniques, finding almost identical results. The final depth measurements obtained by a combined Markov Chain Monte Carlo fit are at 3.6 microns, 1.469 ±0.013 % and 1.448 ±0.013 %; at 4.5 microns, 1.478 ±0.017 % ; at 5.8 microns, 1.549 ±0.015 % and at 8.0 microns 1.535 ±0.011 %. Our results clearly indicate the presence of water in the planetary atmosphere. Our broad band photometric measurements with IRAC prevent us from determining the additional presence of other other molecules such as CO, CO2 and methane for which spectroscopy is needed. While water vapour with a mixing ratio of 10^-4-10^-3 combined with thermal profiles retrieved from the day-side may provide a very good fit to our observations, this data set alone is unable to resolve completely the degeneracy between water abundance and atmospheric thermal profile.

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Une équipe INSU-CNRS de l'Institut d'astrophysique de Paris (CNRS, Université Pierre et Marie Curie) a obtenu le premier spectre optique complet d'une exoplanète, HD209458b. Réalisé à partir des données du télescope spatial Hubble (NASA-ESA), il couvre tout le domaine optique de l'ultraviolet jusqu'à l'infrarouge. En analysant la lumière de l'étoile vue à travers l'atmosphère de sa planète, les astronomes ont ainsi pu déterminer la structure de cette atmosphère. Ils ont noté la présence d'hydrogène et de sodium et éventuellement des oxydes de vanadium et de titane. Le sodium se réparti en plusieurs couches comme les nuages sur Terre, tout étant plus abondant à basse qu'à haute altitude. Par diffusion, le ciel est pourpre et l'étoile au coucher par absorption est cyan. Ces résultats seront prochainement publiés dans Astrophysical Journal.

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Title: New observations of the extended hydrogen exosphere of the extrasolar planet HD209458b
Authors: David Ehrenreich, Alain Lecavelier des Etangs, Guillaume Hébrard, Jean-Michel Désert, Alfred Vidal-Madjar, John C. McConnell, Christopher D. Parkinson, Gilda E. Ballester

Atomic hydrogen escaping from the planet HD209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. However, the Space Telescope Imaging Spectrograph (STIS) used in previous observational studies is no longer available, whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process, and determine the evaporation state of other `hot Jupiters'. Here, we aim to detect the extended hydrogen exosphere of HD209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence for a hydrogen comet-like tail trailing the planet, which size would depend on the escape rate and the amount of ionising radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets. Eight HST orbits are used to observe two transits of HD209458b. Transit light curves are obtained by performing photometry of the unresolved stellar Lyman-alpha emission line during both transits. Absorption signatures of exospheric hydrogen during the transit are compared to light curve models predicting a hydrogen tail. Transit depths of (9.6 ±7.0)% and (5.3 ±10.0)% are measured on the whole Lyman-alpha line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of (8.0 ±5.7)%, in good agreement with previous studies. The extended size of the exosphere confirms that the planet is likely loosing hydrogen to space. Yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass loss rate.

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Title: Determining atmospheric conditions at the terminator of the hot-Jupiter HD209458b
Authors: David K. Sing, A. Vidal-Madjar, A. Lecavelier des Etangs, J.-M. Desert, G. Ballester, D. Ehrenreich

We present a theoretical model fit to the HST/STIS optical transit transmission spectra of HD209458b. In our fit, we use the sodium absorption line profile along with the Rayleigh scattering by H2 to determine the average temperature-pressure profile at the planetary terminator, and infer the abundances of atomic and molecular species. The observed sodium line profile spans an altitude range of ~3,500 km, corresponding to pressures between ~0.001 and 50 mbar in our atmospheric model. We find that the sodium line profile requires condensation at pressures lower than ~3 mbar, presumably into sodium sulphide, depleting atomic sodium only at high altitudes. The condensation of sodium is supported by an observed sudden abundance change, from 2 times solar abundance in the lower atmosphere to 0.2 in the upper atmosphere, within a low temperature region which falls below that of the chemical equilibrium condensation curve of sodium sulphide. Our findings also indicate the presence of a hot atmosphere near stratospheric altitudes corresponding to pressures of ~30 mbar, consistent with that of the observed dayside temperature inversion. In addition, we find a separate higher altitude temperature rise, corresponding to pressures around ~0.01 mbar. This higher altitude temperature rise indicates that absorption by atomic sodium can potentially probe the bottom of the thermosphere, and might possibly be sensitive to the temperature rise linked with atmospheric escape.

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Une équipe internationale(1) comprenant un chercheur du CNRS du Laboratoire d'Astrophysique de Bordeaux vient de montrer que l'enveloppe très étendue d'hydrogène qui entoure l'exoplanète HD209458b trouverait son origine dans le vent stellaire, constitué de protons et électrons éjectés par l'étoile proche. Au contact de l'atmosphère de l'exoplanète, ils se recombineraient pour constituer un nuage d'hydrogène atomique conservant les propriétés cinétiques du vent stellaire. Ce résultat publié dans la revue Nature, permet d'envisager une nouvelle méthode d'étude et de caractérisation des vents stellaires.
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Title: Exoplanet HD 209458b : Evaporation strengthened
Authors: A. Vidal-Madjar, A. Lecavelier des Etangs, J.-M. Desert, G. E. Ballester, R. Ferlet, G. Hebrard, M. Mayor

Following re-analysis of Hubble Space Telescope observations of primary transits of the extrasolar planet HD209458b at Lyman-alpha, Ben-Jaffel (2007, BJ007) claims that no sign of evaporation is observed. Here we show that, in fact, this new analysis is consistent with the one of Vidal-Madjar et al. (2003, VM003) and supports the detection of evaporation. The apparent disagreement is mainly due to the disparate wavelength ranges that are used to derive the transit absorption depth. VM003 derives a (15±4)% absorption depth during transit over the core of the stellar Lyman-alpha line (from -130 km/s to +100 km/s), and this result agrees with the (8.9±2.1)% absorption depth reported by BJ007 from a slightly expanded dataset but over a larger wavelength range (±200 km/s). These measurements agree also with the (5±2)% absorption reported by Vidal-Madjar et al. (2004) over the whole Lyman-alpha line from independent, lower-resolution data. We show that stellar Lyman-alpha variability is unlikely to significantly affect those detections. The HI atoms must necessarily have velocities above the escape velocities and/or be outside the Roche lobe, given the lobe shape and orientation. Absorption by HI in HD209458b's atmosphere has thus been detected with different datasets, and now with independent analyses. All these results strengthen the concept of evaporating hot-Jupiters, as well as the modelisation of this phenomenon.

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