N. P. Gibson, S. Aigrain, F. Pont, D. Sing, J. -M. Désert, T. M. Evans, G. Henry, N. Husnoo, H. Knutson
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\pm0.00048 and 0.15634\pm0.00032, respectively, for visit one
and 0.15716\pm0.00078 and 0.15630\pm0.00037 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.
View original:
http://arxiv.org/abs/1201.6573
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