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Transmission spectroscopy can be used to constrain the properties ofexoplanetary atmospheres. During a transit, the light blocked from theatmosphere of the planet leaves an imprint in the light coming from the star.This has been shown for many exoplanets with different techniques, with bothphotometry and spectroscopy. We aim at testing chromatic line-profiletomography as a new tool to investigate exoplanetary atmospheres. The signalimprinted on the cross-correlation function (CCF) by a planet transiting itsstar is dependent on the planet-to-star radius ratio. We want to verify if theprecision reachable on the CCF obtained from a subset of the spectral orders ofthe HARPS spectrograph is enough to discriminate the radius of a planet atdifferent wavelengths. Methods. We analyze HARPS archival data of threetransits of HD 189733b. We divide the HARPS spectral range in 7 broadbands,calculating for each band the ratio between the area of the out-of-transit CCFand the area of the signal imprinted by the planet on it during the full partof the transit. We take into account the effect of the limb darkening using thetheoretical coefficients of a linear law. Averaging the results of threedifferent transits allows us to obtain a good quality broadband transmissionspectrum of HD 189733b, with a precision greater than that of the chromatic RMeffect. Results. We proved chromatic line-profile tomography to be aninteresting way to reveal broadband transmission spectra of exoplanets: ouranalysis of the atmosphere of HD 189733b is in agreement with other ground- andspace-based observations. The independent analysis of different transits putsin evidence the probability that stellar activity plays a role in the extractedtransmission spectrum. Care has thus to be taken when claiming for Rayleighscattering in the atmosphere of exoplanets orbiting active stars using only onetransit.

Chromatic line-profile tomography to reveal exoplanetary atmospheres: application to HD 189733b