Retrieving day- and nightside atmospheric properties of the ultra-hot Jupiter TOI-2109b

Detection of Fe and CO emission lines and evidence for inefficient heat transport

¹ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany
² Exzellenzcluster Origins, Boltzmannstrasse 2, 85748 Garching bei München, Germany
³ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ Namzitu astro, 31130 Quint-Fonsegrives, France
⁷ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Instituto de Astrofísica de Canarias (IAC), Calle Vía Lactea s/n, 38200 La Laguna, Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna, 38026 La Laguna, Tenerife, Spain
¹⁰ Centro de Astrobiología (CAB), CSIC-INTA, Camino bajo del castillo s/n, Campus ESAC, 28692 Villanueva de la Cañada, Madrid, Spain
¹¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹⁴ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
¹⁵ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, c/ de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
¹⁶ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
¹⁷ Centro Astronómico Hispano en Andalucía, Observatorio de Calar Alto, Sierra de los Filabres, 04550 Gérgal, Spain
¹⁸ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁹ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai 400005, India
²⁰ ARTORG Center for Biomedical Engineering Research, University of Bern, Murtenstrasse 50, 3008 Bern, Switzerland
²¹ University College London, Department of Physics & Astronomy, Gower St, London, WC1E 6BT, UK
²² Astronomy & Astrophysics Group, Department of Physics, University of Warwick, Coventry CV4 7AL, UK
²³ Departamento de Física de la Tierra y Astrofísica and IPARCOS-UCM (Instituto de Física de Partículas y del Cosmos de la UCM), Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
²⁴ INAF, Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
²⁵ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
²⁶ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²⁷ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

^★ Corresponding author: david.cont@lmu.de

Received: 17 March 2025
Accepted: 17 April 2025

Abstract

The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500 and 4500 K, an unexplored thermal regime that separates KELT-9b, the hottest planet yet discovered, from all other currently known gas giants. To study the thermochemical structure of TOI-2109b’s atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CAHA/CARMENES and VLT/CRIRES⁺. By applying the cross-correlation technique to the high-resolution spectra, we identified the emission signatures of Fe I (S/N = 4.3) and CO (S/N = 6.3), as well as a thermal inversion layer in the dayside atmo-sphere; no significant H₂O signal was detected from the dayside. None of the analyzed species were detectable from the nightside atmosphere. We applied a Bayesian retrieval framework that combines high-resolution spectroscopy with photometric measurements to constrain the dayside atmospheric parameters and derive upper limits for the nightside hemisphere. The dayside thermal inversion extends from approximately 3200 to 4600 K, with an atmospheric metallicity consistent with that of the host star (0.36 dex). Only weak constraints could be placed on the C/O ratio, with a lower limit of 0.15. The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes in the atmosphere. An upper temperature limit of approximately 2400 K and a maximum atmospheric temperature gradient of about 700 K/log bar could be derived for the planetary nightside. Comparison of the retrieved dayside temperature-pressure profile with theoretical models, the absence of strong atmospheric dynamics, and significant differences in the thermal constraints between the day- and nightside hemispheres suggest a limited heat transport efficiency across the planetary atmosphere. Overall, our results place TOI-2109b in a transitional regime between the UHJs below the thermal gap, which show both CO and H₂O emission lines, and KELT-9b, where molecular features are largely absent.