The GAPS Programme at TNG

LXVII. Detection of water and preliminary characterisation of the atmospheres of the two hot Jupiters: KELT-8 b and KELT-23 Ab^★

¹ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
³ Department of Physics, University of Turin, Via Pietro Giuria 1, 10125 Torino, Italy
⁴ Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁵ ESO – European Southern Observatory, Alonso de Cordova, 3107 Vitacura, Santiago, Chile
⁶ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja (La Palma), Canary Islands, Spain
⁹ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
¹⁰ Departamento de Astrofísica, Univ. de La Laguna, Av. del Astrofísico Francisco Sánchez s/n, 38205 La Laguna (Tenerife), Canary Islands, Spain
¹¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
¹² INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹³ Department of Physics and Chemistry, University of Palermo, Piazza Marina 61, 90133 Palermo, Italy
¹⁴ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁵ Department of Physics and Astronomy Galileo Galilei, University of Padua, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

^★★ Corresponding author; mario.basilicata@inaf.it

Received: 24 October 2024
Accepted: 6 February 2025

Abstract

Context. Hot Jupiters are among the most suitable targets for atmospheric studies. Expanding the number of hot gaseous giant planets with atmospheric characterisations can improve our understanding of the chemical-physical properties of their atmospheres, as well as the formation and evolution of these extreme planets.

Aims. In this work, we use high-resolution spectroscopy in the near-infrared (NIR) to search for chemical signatures in the atmospheres of the two hot Jupiters KELT-8 b (T_eq = 1675₋₅₅⁺⁶¹ K) and KELT-23 Ab (T_eq = 1561 ± 20 K) and present a first characterisation of their atmospheric properties.

Methods. We measured the transmission spectrum of each target with the near-infrared (NIR) high-resolution spectrograph GIANO-B at the TNG. We searched for atmospheric signals by cross-correlating the data with synthetic transmission spectra. To characterise the chemical-physical properties of the atmospheres of both planets, we ran two different atmospheric retrievals for each dataset: a retrieval assuming chemical equilibrium and a ‘free-chemistry’ retrieval, in which the abundance of each molecule could vary freely.

Results. We detect water vapour (H₂O) in the atmospheres of both KELT-8 b and KELT-23 Ab with a signal-to-noise ratio of S/N = 6.6 and S/N = 4.2, respectively. The two retrievals indicate a water-rich atmosphere for both targets. In the case of KELT-8 b, we determine a water volume mixing ratio of log₁₀ (VMR_H2O) = −2.07_−0.72^+0.53, a metallicity of [M/H = 0.77_−0.89^+0.61 dex, and a sub-solar C/O ratio (C/O ≤0.30, at 2 σ). For KELT-23 Ab, we find log₁₀ (VMR_H2O) = −2.26_−1.24^+0.75, [M/H]= −0.42_−1.35^+1.56 dex, and C/O ratio ≤0.78 (at 2 σ). The constraints on the metallicity and C/O ratio are based on the assumption of chemical equilibrium. Comparing these atmospheric chemical properties with those of the host stars, we suggest that for both planets, the accretion of gaseous material occurred within the H₂O snowline in a pebble-rich disk enriched in oxygen due to sublimation of water ice from the inward-drifting pebbles.

Conclusions. We investigated the atmospheres of KELT-8 b and KELT-23 Ab for the first time, finding water vapour in both of them and placing first constraints on their properties. These two planets are promising targets for future high- and low-resolution observations.