Search for water vapor in the high-resolution transmission spectrum of HD 189733b in the visible

¹ Observatoire Astronomique de l’Université de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
e-mail: romain.allart@unige.ch
² Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy

Received: 17 March 2017
Accepted: 25 May 2017

Abstract

Context. 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 in a number of hot Jupiters. 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.

Aims. We aim to search for water vapor in the atmosphere of the exoplanet HD 189733b using a high-resolution transmission spectrum in the visible obtained with HARPS.

Methods. We used the atmospheric transmission code Molecfit to correct for telluric absorption features. Then we computed the high-resolution transmission spectrum of the planet using three transit datasets. We finally searched for water vapor absorption in the water band around 6500 Å using a cross-correlation technique that combines the signal of 600–900 individual lines.

Results. Telluric features are corrected to the noise level. We place a 5-σ upper limit of 100 ppm on the strength of the 6500 Å water vapor band. The 1-σ precision of 20 ppm on the transmission spectrum demonstrates that space-like sensitivity can be achieved from the ground, even for a molecule that is a strong telluric absorber.

Conclusions. This approach opens new possibilites for the detection of various atomic and molecular species with future instruments such as ESPRESSO at the VLT. Extrapolating from our results, we show that only one transit with ESPRESSO would be sufficient to detect water vapor on HD 189733b-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 (cloud and/or hazes).