  We present the analysis of high-resolution optical spectra of four transits
of 55Cnc e, a low-density, super-Earth that orbits a nearby Sun-like star in
under 18 hours. The inferred bulk density of the planet implies a substantial
envelope, which, according to mass-radius relationships, could be either a
low-mass extended or a high-mass compact atmosphere. Our observations
investigate the latter scenario, with water as the dominant species. We take
advantage of the Doppler cross-correlation technique, high-spectral resolution
and the large wavelength coverage of our observations to search for the
signature of thousands of optical water absorption lines. Using our
observations with HDS on the Subaru telescope and ESPaDOnS on the
Canada-France-Hawaii Telescope, we are able to place a 3-sigma lower limit of
10 g/mol on the mean-molecular weight of 55Cnc e's water-rich (volume mixing
ratio >10%), optically-thin atmosphere, which corresponds to an atmospheric
scale-height of ~80 km. Our study marks the first high-spectral resolution
search for water in a super-Earth atmosphere and demonstrates that it is
possible to recover known water-vapour absorption signals, in a nearby
super-Earth atmosphere, using high-resolution transit spectroscopy with current
ground-based instruments.
