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

Abstract

The atmospheric characterisation of hot and warm Neptune-size exoplanets is challenging due to their small radius and atmospheric scale height. The warm-Neptune HAT-P-11b is a remarkable target for such characterisation due to 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 8 molecular species in the atmosphere of HAT-P-11b at high spectral resolution in the near-infrared. The planetary system was reviewed by analysing transits and occultations of HAT-P-11b from the Kepler data set as well as HIRES at Keck archival radial-velocity (RV) 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-11b, we observed 4 transits of this target with GIANO-B at TNG. We find that the long-period ($P\sim 9.3$ years) RV signal previously attributed to planet HAT-P-11c 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-11b, we measure a radius $R_{\mathrm{p}}=0.4466\pm 0.0059R_{\mathrm{J}}$, a mass $M_{\mathrm{p}}=0.0787\pm 0.0048M_{\mathrm{J}}$, and an eccentricity $e=0.2577_{-0.0025}^{+0.0033}$, in accordance with values in the literature. Probing its atmosphere, we detect $N{H}_3$ (S/N$=5.3$, significance$=5.0\ddot{I}ƒ$) and confirm the presence of $H_{2}O$ (S/N$=5.1$, significance$=3.4\ddot{I}ƒ$). We also tentatively detect the signal of $C{O}_2$ (S/N$=3.0$, significance$=3.2\ddot{I}ƒ$) and $C{H}_4$ (S/N$=4.8$, significance$=2.6\ddot{I}ƒ$), whose presence need to be confirmed by further observations.

25 pages, 19 figures, accepted for publication in Astronomy & Astrophysics (A&A) journal. Version corrected by the language editor, title edited