The GAPS programme at TNG

XLV. HI Balmer lines transmission spectroscopy and NLTE atmospheric modelling of the ultra-hot Jupiter KELT-20b/MASCARA-2b

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: Luca.Fossati@oeaw.ac.at
² DISAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
³ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁴ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125, Torino, Italy
⁵ Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
⁶ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomia s/n, 18008 Granada, Spain
⁷ INAF - Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁸ Lunar and Planetary Laboratory, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85721-0092, USA
⁹ INAF - Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁰ INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹¹ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
¹² INAF - Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
¹³ INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
¹⁴ Instituto de Astrofisica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹⁵ Fundación Galileo Galilei - INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
¹⁶ Dipartimento di Fisica e Astronomia “Galileo Galilei” - Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁷ Department of Physics, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁹ INAF - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
²⁰ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy

Received: 2 May 2023
Accepted: 28 June 2023

Abstract

Aims. We aim to extract the transmission spectrum of the HI Balmer lines of the ultra-hot Jupiter (UHJ) KELT-20b/MASCARA-2b from observations and to further compare the results with what was obtained through forward modelling, accounting for non-local thermodynamic equilibrium (NLTE) effects.

Methods. We extracted the line profiles from six transits obtained with the HARPS-N high-resolution spectrograph attached to the Telescopio Nazionale Galileo telescope. We computed the temperature-pressure (TP) profile employing the HELIOS code in the lower atmosphere and the CLOUDY NLTE code in the middle and upper atmosphere. We further used CLOUDY to compute the theoretical planetary transmission spectrum in LTE and NLTE for comparison with observations.

Results. We detected the Hα (0.79±0.03%; 1.25 R_p), Ηβ (0.52±0.03%; 1.17 R_p), and Ηγ (0.39±0.06%; 1.13 R_p) lines, and we detected the Ηδ line at almost 4σ (0.27±0.07%; 1.09 R_p). The models predict an isothermal temperature of ≈2200 K at pressures >10⁻² bar and of ≈7700 K at pressures <10⁻⁸ bar, with a roughly linear temperature rise in between. In the middle and upper atmosphere, the NLTE TP profile is up to ~3000 K hotter than in LTE. The synthetic transmission spectrum derived from the NLTE TP profile is in good agreement with the observed HI Balmer line profiles, validating our obtained atmospheric structure. Instead, the synthetic transmission spectrum derived from the LTE TP profile leads to significantly weaker absorption compared to the observations.

Conclusions. Metals appear to be the primary agents leading to the temperature inversion in UHJs, and the impact of NLTE effects on them increases the magnitude of the inversion. We find that the impact of NLTE effects on the TP profile of KELT-20b/MASCARA-2b is larger than for the hotter UHJ KELT-9b, and thus NLTE effects might also be relevant for planets cooler than KELT-20b/MASCARA-2b.