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In the framework of the GAPS project, we observed the planet-hosting starKELT-9 (A-type star, VsinI$\sim$110 km/s) with the HARPS-N spectrograph at theTNG. In this work we analyse the spectra and the extracted radial velocities(RVs), to constrain the physical parameters of the system and to detect theplanetary atmosphere of KELT-9b. We extracted from the high-resolution opticalspectra the mean stellar line profiles with an analysis based on the LeastSquare Deconvolution technique. Then, we computed the stellar RVs with a methodoptimized for fast rotators, by fitting the mean stellar line profile with apurely rotational profile instead of using a Gaussian function. The new spectraand analysis led us to update the orbital and physical parameters of thesystem, improving in particular the value of the planetary mass to $M_{\rmp}=2.88\pm0.35\,M_{\rm Jup}$. We discovered an anomalous in-transit RVdeviation from the theoretical Rossiter-McLaughlin effect solution, calculatedfrom the projected spin-orbit angle $\lambda=-85.78\pm0.46$ degrees measuredwith Doppler tomography. We prove that this deviation is caused by theplanetary atmosphere of KELT-9b, thus we name this effect AtmosphericRossiter-McLaughlin effect. By analysing the magnitude of the RV anomaly, weobtained information on the extension of the planetary atmosphere as weightedby the model used to retrieve the stellar mean line profiles, which is up to$1.22\pm0.02\,R_{\rm p}$. The Atmospheric Rossiter-McLaughlin effect will beobservable for other exoplanets whose atmosphere has non-negligible correlationwith the stellar mask used to retrieve the RVs, in particular ultra-hotJupiters with iron in their atmosphere. The duration and amplitude of theeffect will depend not only on the extension of the atmosphere, but also on thein-transit planetary RVs and on the projected rotational velocity of the parentstar.

The GAPS Programme with HARPS-N at TNG XIX. Atmospheric Rossiter-McLaughlin effect and improved parameters of KELT-9b