2024A&A...686A.127B

Context. 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).
Aims. 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.
Methods. 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.
Results. We find that the long-period radial-velocity signal previously attributed to the HAT-P-11 c planet (P ∼ 9.3 yr; M_p sin i ~ 1.6 M_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 R_p = 0.4466 ± 0.0059 R_J, a mass of M_p = 0.0787 ± 0.0048 M_J, a bulk density of ρ_p = 1.172 ± 0.085 g cm^–3, and an orbital eccentricity of e = 0.2577_–0.0025^+0.0033. 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 σ and 5.0 σ, 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 σ and 2.6 σ, respectively.
Conclusions. We revisit the HAT-P-11 planetary system, confirm the presence of H₂O, 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.