  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 corner stone for hot Jupiter models. In this paper,
we report the day-side emission spectrum of HD 189733b in the wavelength range
1.1 to 1.7 $\mu$m obtained with the Hubble Space Telescope Wide Field Camera 3
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$\pm$39 ppm. The resulting spectrum shows marginal
evidence for water vapor absorption, but can also be well explained by a
blackbody spectrum. However, the combination of these WFC3 data with previous
Spitzer photometric observations is best explained by a day-side atmosphere of
HD 189733b with no thermal inversion and a nearly solar or sub-solar H$_2$O
abundance in a cloud-free atmosphere. Alternatively, this apparent sub-solar
abundance may be the result of clouds or hazes which future studies need to
investigate.
