The GAPS programme at TNG

LX. Atmospheric characterisation of KELT-9 b via single-line analysis: Detection of six H I Balmer lines, Na I, Ca I, Ca II, Fe I, Fe II, Mg I, Ti II, Sc II, and Cr II^★

¹ INAF – Palermo Astronomical Observatory, Piazza del Parlamento, 1, 90134 Palermo, Italy
² University of Palermo, Department of Physics and Chemistry “Emilio Segrè”, Via Archirafi 36, Palermo, Italy
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁵ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁸ Dipartimento di Fisica, Universitá degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy
⁹ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123, Catania, Italy
¹⁰ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹² Fundación Galileo Galilei-INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
¹³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
¹⁴ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy

^★★ Corresponding author; mattia.darpa@inaf.it

Received: 25 January 2024
Accepted: 13 August 2024

Abstract

Aims. We analysed six primary transits of the ultra-hot Jupiter KELT-9 b obtained with the HARPS-N high-resolution spectrograph in the context of the Global Architecture of Planetary Systems (GAPS2) project, to characterise the atmosphere via single-line analysis.

Methods. We extracted the transmission spectrum of each individual line by comparing the master out-of-transit spectrum with the in-transit spectra and computing the weighted average of the tomography in the planet reference frame. We corrected for the centre-to-limb variation and the Rossiter-McLaughlin effect by modelling the region of the star disc obscured by the planet during the transit and subtracting it from the master-out spectrum.

Results. We detected all six observable lines of the Balmer series within the HARPS-N wavelength range, from Hα to Hζ, with a significance exceeding 5σ. We also focussed on metal species, detecting Na I, Ca I, Ca II, Fe I, Fe II, Mg I, Ti II, Sc II, and Cr II lines. This is the first detection in the atmosphere of an exoplanet of Hϵ and Hζ lines, as well as of individual lines of Sc II and Cr II. Our detections are supported by a comparison with published synthetic transmission spectra of KELT-9b obtained accounting for non-local thermodynamic equilibrium effects. The results underline the presence of a systematic blueshift due to night-side to day-side winds.

Conclusions. The single-line analysis allowed us not only to assess the presence of atomic species in the atmosphere of KELT-9 b, but also to further characterise the local stratification of the atmosphere. Coupling the height distribution of the detected species with the velocity shift retrieved, we acknowledged the height distribution of night-side to day-side winds. Moreover, the study of the rotational broadening of the different species supports the prediction of a tidally locked planet rotating as a rigid body.