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^★

¹ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
e-mail: mario.basilicata@inaf.it
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
³ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁴ Department of Physics, University of Turin, Via Pietro Giuria 1, 10125 Torino, Italy
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁷ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Spain
⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
⁹ Astrophysics Group, Keele University, Keele ST5 5BG, UK
¹⁰ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlßtrasse 6, 8042 Graz, Austria

Received: 4 August 2023
Accepted: 29 February 2024

Abstract

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.