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Abstract:
We observed the planet-hosting star KELT-9 (A-type star, vsini ~110 km/s) in the framework of the GAPS project (Global Architecture of Planetary Systems). GAPS is a long-term italian program using the HARPS-N and GIANO-B spectrographs at the TNG telescope in order to detect and characterize planetary systems, with a strong focus on the study of the exoplanets' atmospheres. We extracted from the high- resolution optical HARPS-N spectra of KELT-9 the mean stellar line profiles with a custom analysis based on the Least Square Deconvolution technique. Then, we computed the stellar radial velocities with a method developed for fast rotators, by fitting the mean stellar line profile with a purely rotational profile instead of using a Gaussian function. The new spectra and analysis led us to update the orbital and physical parameters of the system. The former depends on the improvement on the Kstar value, that we measured with an 8-σ significance, a remarkable result for such a fast rotating star. We discovered an anomalous in- transit radial velocity deviation from the theoretical Rossiter- McLaughlin effect solution, previously calculated from the known spin- orbit angle obtained from the line profile tomography. We prove that this deviation is caused by the planetary atmosphere of KELT-9b, thus we name this effect Atmospheric Rossiter-McLaughlin effect. By analysing the magnitude of the radial velocity anomaly, we obtained information on the extension of the planetary atmosphere as weighted by the model used to retrieve the stellar mean line profiles. The Atmospheric Rossiter- McLaughlin effect will be observable for other exoplanets whose atmosphere has non-negligible correlation with the stellar mask used to retrieve the radial velocities, in particular ultra-hot Jupiters with iron in their atmospheres. The duration and amplitude of the effect will depend not only on the extension of the atmosphere, but also on the in- transit planetary radial velocities and on the projected rotational velocity of the parent star, and this could led to strange in-transit RV variations.
