The GAPS Programme at TNG

XLI. The climate of KELT-9b revealed with a new approach to high-spectral-resolution phase curves

¹ INAF – Osservatorio Astrofísico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
e-mail: lorenzo.pino@inaf.it
² Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
³ INAF – Osservatorio Astrofísico di Torino, Via Osservatorio 20, 10025, Pino Torinese, Italy
⁴ Centre for Exoplanets and Habitability, University of Warwick, Coventry, CV4 7AL, UK
⁵ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
⁶ INAF – Osservatorio Astronomico di Padova, Padova 35122, Italy
⁷ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
⁸ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁹ INAF – Osservatorio Astrofísico di Catania, Via Santa Sofia 78, 95123 Catania, Italy
¹⁰ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
¹² INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹³ INAF – IAPS Istituto di Astrofísica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁴ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy
¹⁵ Dip. di Fisica e Astronomia Galileo Galilei – Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
¹⁶ INAF – Osservatorio di Cagliari, via della Scienza 5, 09047 Selargius, Italy
¹⁷ Aix Marseille Univ, CNRS, CNES, LAM, 13388 Marseille, France
¹⁸ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Brena Baja, TF, Spain

Received: 25 July 2022
Accepted: 25 September 2022

Abstract

Aims. We present a novel method for studying the thermal emission of exoplanets as a function of orbital phase at very high spectral resolution, and use it to investigate the climate of the ultra-hot Jupiter KELT-9b.

Methods. We combine three nights of HARPS-N and two nights of CARMENES optical spectra, covering orbital phases between quadratures (0.25 < φ < 0.75), when the planet shows its day-side hemisphere with different geometries. We co-add the signal of thousands of Fe I lines through cross-correlation, which we map to a likelihood function. We investigate the phase-dependence of two separate observable quantities, namely (i) the line depths of Fe I and (ii) their Doppler shifts, introducing a new method that exploits the very high spectral resolution of our observations.

Results. We confirm a previous detection of Fe I emission, and demonstrate a precision of 0.5 km s⁻¹ on the orbital properties of KELT-9b when combining all nights of observations. By studying the phase-resolved Doppler shift of Fe I lines, we detect an anomaly in the planet's orbital radial velocity well-fitted with a slightly eccentric orbital solution (e = 0.016 ± 0.003, ω = 150₋₁₁⁺¹³°, 5σ preference). However, we argue that this anomaly is caused by atmospheric circulation patterns, and can be explained if neutral iron gas is advected by day-to-night atmospheric wind flows of the order of a few km s⁻¹. We additionally show that the Fe I emission line depths are symmetric around the substellar point within 10° (2σ), possibly indicating the lack of a large hot-spot offset at the altitude probed by neutral iron emission lines. Finally, we do not obtain a significant preference for models with a strong phase-dependence of the Fe I emission line strength. We show that these results are qualitatively compatible with predictions from general circulation models (GCMs) for ultra-hot Jupiter planets.

Conclusions. Very high-resolution spectroscopy phase curves are of sufficient sensitivity to reveal a phase dependence in both the line depths and their Doppler shifts throughout the orbit. They constitute an under-exploited treasure trove of information that is highly complementary to space-based phase curves obtained with HST and JWST, and open a new window onto the still poorly understood climate and atmospheric structure of the hottest planets known to date.