* Astronomy

Title: Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
Authors: A. Lecavelier des Etangs, V. Bourrier, P.J. Wheatley, H. Dupuy, D. Ehrenreich, A. Vidal-Madjar, G. Hébrard, G.E. Ballester, J.-M. Désert, R. Ferlet, D.K. Sing

Atmospheric escape has been detected from the exoplanet HD 209458b through transit observations of the hydrogen Lyman-alpha line. Here we present spectrally resolved Lyman-alpha transit observations of the exoplanet HD 189733b at two different epochs. These HST/STIS observations show for the first time, that there are significant temporal variations in the physical conditions of an evaporating planetary atmosphere. While atmospheric hydrogen is not detected in the first epoch observations, it is observed at the second epoch, producing a transit absorption depth of 14.4±3.6% between velocities of -230 to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from radiation pressure alone and require an additional acceleration mechanism, such as interactions with stellar wind protons. The observed absorption can be explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9 g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K.
An X-ray flare from the active star seen with Swift/XRT 8 hours before the second-epoch observation supports the idea that the observed changes within the upper atmosphere of the planet can be caused by variations in the stellar wind properties, or by variations in the stellar energy input to the planetary escaping gas (or a mix of the two effects). These observations provide the first indication of interaction between the exoplanet's atmosphere and stellar variations.

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Title: Temperature-Pressure Profile of the hot Jupiter HD 189733b from HST Sodium Observations: Detection of Upper Atmospheric Heating
Authors: Catherine M. Huitson, David K. Sing, Alfred Vidal-Madjar, Gilda E. Ballester, Alain Lecavelier des Etangs, Jean-Michel Désert, Frédéric Pont

We present transmission spectra of the hot Jupiter HD 189733b taken with the Space Telescope Imaging Spectrograph aboard HST. The spectra cover the wavelength range 5808-6380 Ang with a resolving power of R=5000. We detect absorption from the NaI doublet within the exoplanet's atmosphere at the 9 sigma confidence level within a 5 Ang band (absorption depth 0.09 ±0.01%) and use the data to measure the doublet's spectral absorption profile. We detect only the narrow cores of the doublet. The narrowness of the feature could be due to an obscuring high-altitude haze of an unknown composition or a significantly sub-solar NaI abundance hiding the line wings beneath a H2 Rayleigh signature. We compare the spectral absorption profile over 5.5 scale heights with model spectral absorption profiles and constrain the temperature at different atmospheric regions, allowing us to construct a vertical temperature profile. We identify two temperature regimes; a 1280 ±240 K region derived from the NaI doublet line wings corresponding to altitudes below ~ 500 km, and a 2800 ±400 K region derived from the NaI doublet line cores corresponding to altitudes from ~ 500-4000 km. The zero altitude is defined by the white-light radius of Rp/Rstar=0.15628 ±0.00009. The temperature rises with altitude, which is likely evidence of a thermosphere. The absolute pressure scale depends on the species responsible for the Rayleigh signature and its abundance. We discuss a plausible scenario for this species, a high-altitude silicate haze, and the atmospheric temperature-pressure profile that results. In this case, the high altitude temperature rise for HD 189733b occurs at pressures of 10^-5 to 10^-8 bar.

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A Molleweide projection of the 2D infrared map of HD 189733b with lmax = 1; latitude and longitude lines are spaced every 30 deg. Coordinates are centred on the sub-stellar point and intensity values are relative (white is brightest, black is 30.2% as bright).

Title: Probing the haze in the atmosphere of HD 189733b with HST/WFC3 transmission spectroscopy
Authors: N. P. Gibson (1), S. Aigrain (1), F. Pont (2), D. Sing (2), J.-M. Désert (3), T. M. Evans (1), G. Henry (4), N. Husnoo (2), H. Knutson (5) ((1) University of Oxford, (2) University of Exeter, (3) Harvard-Smithsonian CfA, (4) Tennessee State University, (5) Caltech)

We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting exoplanet HD 189733b, using Wide Field Camera 3. This consists of time-series spectra of two transits, used to measure the wavelength dependence of the planetary radius. These observations aim to test whether the Rayleigh scattering haze detected at optical wavelengths extends into the near-infrared, or if it becomes transparent leaving molecular features to dominate the transmission spectrum. Due to saturation and non-linearity affecting the brightest (central) pixels of the spectrum, light curves were extracted from the blue and red ends of the spectra only, corresponding to wavelength ranges of 1.099-1.168 um and 1.521-1.693 um, respectively, for the first visit, and 1.082-1.128 um and 1.514-1.671 um for the second. The light curves were fitted using a Gaussian process model to account for instrumental systematics whilst simultaneously fitting for the transit parameters. This gives values of the planet-to-star radius ratio for the blue and red light curves of 0.15650±0.00048 and 0.15634±0.00032, respectively, for visit one and 0.15716±0.00078 and 0.15630±0.00037 for visit 2 (using a quadratic limb darkening law). The planet-to-star radius ratios measured in both visits are consistent, and we see no evidence for the drop in absorption expected if the haze that is observed in the optical becomes transparent in the infrared. This tentatively suggests that the haze dominates the transmission spectrum of HD 189733b into near-infrared wavelengths, although more robust observations are required to provide conclusive evidence.

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