The GAPS Programme at TNG LV. Multiple Molecular Species in the Atmosphere of HAT-P-11 B and Review of the HAT-P-11 Planetary System

The atmospheric characterisation of hot and warm Neptune-size exoplanets is challenging mainly due to their relatively small radius and atmospheric scale height, which reduce the amplitude of atmospheric spectral features. The warm-Neptune HAT-P-11\,b is a remarkable target for atmospheric characterisation because of the large brightness of its host star ($V=9.46$\,mag; $H=7.13$\,mag). The aims of this work are to review the main physical and architectural properties of the HAT-P-11 planetary system, and to probe the presence of eight molecular species in the atmosphere of HAT-P-11\,b through near-infrared (NIR) high-resolution transmission spectroscopy. We reviewed the physical and architectural properties of the HAT-P-11 planetary system by analysing transits and occultations of HAT-P-11\,b from the Kepler data set as well as HIRES at Keck archival radial-velocity data. We modelled the latter with Gaussian-process regression and a combined quasi-periodic and squared-exponential kernel to account for stellar variations on both (short-term) rotation and (long-term) activity-cycle timescales. In order to probe the atmospheric composition of HAT-P-11\,b, we observed four transits of this target with the NIR GIANO-B at TNG spectrograph and cross-correlated the data with template atmospheric transmission spectra. We find that the long-period radial-velocity signal previously attributed to the HAT-P-11\,c planet ($P 9.3$\,years; p i J $; $e 0.6$) is more likely due to the stellar magnetic activity cycle. Nonetheless, the Hipparcos-Gaia difference in the proper-motion anomaly suggests that an outer-bound companion might still exist. For HAT-P-11\,b, we measure a radius of p J $, a mass of $M_ p J $, a bulk density of $ p $, and an orbital eccentricity of $e=0.2577^ $. These values are compatible with those from the literature. Probing its atmosphere, we detect the presence of two molecular species H2O and NH3 with a S/N of $5.1$ and $5.3$, and a significance of $3.4\ and $5.0\ respectively. We also tentatively detect the presence of CO2 and CH4 with a S/N of $3.0$ and $4.8$, and a significance of $3.2\ and $2.6\ respectively. We revisit the HAT-P-11 planetary system, confirm the presence of water vapour, and report the detection of NH3 in the atmosphere of HAT-P-11\,b, also finding hints for the presence of CO2 and CH4 that need to be confirmed by further observations.