WATER VAPOR IN THE SPECTRUM OF THE EXTRASOLAR PLANET HD 189733b. II. THE ECLIPSE

Spectroscopic observations of exoplanets are crucial to infer the composition and properties of their atmospheres. HD 189733b is one of the most extensively studied exoplanets and is a cornerstone for hot Jupiter models. In this paper, we report the dayside emission spectrum of HD 189733b in the wavelength range 1.1-1.7 mu m obtained with the Hubble Space Telescope Wide Field Camera 3 (WFC3) in spatial scan mode. The quality of the data is such that even a straightforward analysis yields a high-precision Poisson noise-limited spectrum: the median 1 sigma uncertainty is 57 ppm per 0.02 mu m bin. We also build a white-light curve correcting for systematic effects and derive an absolute eclipse depth of 96 +/- 39 ppm. The resulting spectrum shows marginal evidence for water vapor absorption, but can also be well explained by a blackbody spectrum. However, the combination of these WFC3 data with previous Spitzer photometric observations is best explained by a dayside atmosphere of HD 189733b with no thermal inversion and a nearly solar or subsolar H2O abundance in a cloud-free atmosphere. Alternatively, this apparent subsolar abundance may be the result of clouds or hazes that future studies need to investigate.