Title: Search for water vapor in the high-resolution transmission spectrum of HD189733b in the visible,
Abstract: Ground-based telescopes equipped with state-of-the-art spectrographs are able
to obtain high-resolution transmission and emission spectra of exoplanets that
probe the structure and composition of their atmospheres. Various atomic and
molecular species, such as Na, CO, H2O have been already detected. Molecular
species have been observed only in the near-infrared while atomic species have
been observed in the visible. In particular, the detection and abundance
determination of water vapor bring important constraints to the planet
formation process. We search for water vapor in the atmosphere of the exoplanet
HD189733b using a high-resolution transmission spectrum in the visible obtained
with HARPS. We use Molecfit to correct for telluric absorption features. Then
we compute the high-resolution transmission spectrum of the planet using 3
transit datasets. We finally search for water vapor absorption using a
cross-correlation technique that combines the signal of 800 individual lines.
Telluric features are corrected to the noise level. We place a 5-sigma upper
limit of 100 ppm on the strength of the 6500 A water vapor band. The 1-sigma
precision of 20 ppm on the transmission spectrum demonstrates that space-like
sensitivity can be achieved from the ground. This approach opens new
perspectives to detect various atomic and molecular species with future
instruments such as ESPRESSO at the VLT. Extrapolating from our results, we
show that only 1 transit with ESPRESSO would be sufficient to detect water
vapor on HD189733b-like hot Jupiter with a cloud-free atmosphere. Upcoming
near-IR spectrographs will be even more efficient and sensitive to a wider
range of molecular species. Moreover, the detection of the same molecular
species in different bands (e.g. visible and IR) is key to constrain the
structure and composition of the atmosphere, such as the presence of Rayleigh
scattering or aerosols.