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

DOI
Publisher DOI
Abstract
The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500 K 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 structure of TOI-2109b's atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CARMENES and CRIRES$^+$. By applying the cross-correlation technique, we identified the emission signatures of Fe I and CO, as well as a thermal inversion layer in the dayside atmosphere; no significant H$_2$O 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 3200 K 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 ($>$ 0.15). The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes. An upper temperature limit of 2400 K and a maximum atmospheric temperature gradient of 700 K/log bar could be derived for the nightside. Comparison of the retrieved dayside T-p 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 H$_2$O emission lines, and KELT-9b, where molecular features are largely absent....
Date Issued
2025
Publication Type
Article
Subject(s)
Subjects
techniques: spectroscopic
•
planets and satellites: atmospheres
•
planets and satellites: individual: TOI-2109b
Language(s)
en
Author(s)
Cont, D.
Nortmann, L.
Lesjak, F.
Yan, F.
Shulyak, D.
Lavail, A.
Stangret, M.
Pallé, E.
Amado, P. J.
Caballero, J. A.
Hatzes, A.
Henning, Th.
Piskunov, N.
Quirrenbach, A.
Reiners, A.
Ribas, I.
Agüí Fernández, J. F.
Boldt-Christmas, L.
Chaturvedi, P.
Czesla, S.
Hahlin, A.
Kochukhov, O.
Marquart, T.
Molaverdikhani, K.
Montes, D.
Morello, G.
Orell-Miquel, J.
Rains, A. D.
Rengel, M.
Schweitzer, A.
Sánchez-López, A.
Seemann, U.
Additional Credits
Journal
Astronomy & Astrophysics
Publisher
EDP Sciences
ISSN
0004-6361
1432-0746
Related URL(s)
https://doi.org/10.48550/arXiv.2504.15757
Access(Rights)
open.access
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