L. Bazinet, R. Allart, B. Benneke, S. Pelletier, J. P. Wardenier, N. J. Cook, T. Forveille, L. D. Nielsen, K. Al Moulla, E. Artigau, F. Baron, S. C. C. Barros, X. Bonfils, F. Bouchy, M. Bryan, B. L. Canto Martins, R. Cloutier, N. B. Cowan, D. Brito de Freitas, J. R. de Medeiros, X. Delfosse, R. Doyon, X. Dumusque, D. Ehrenreich, J. I. González Hernández, D. Lafrenière, I. de Castro Leão, C. Lovis, L. Malo, C. Melo, L. Mignon, C. Mordasini, F. Pepe, R. Rebolo, J. F. Rowe, N. C. Santos, D. Ségransan, A. Suárez Mascareño, S. Udry, D. Valencia, G. A. Wade, M. Abreu, J. L. A. Aguiar, G. Allain, T. Arial, H. Auger, N. Blind, D. Bohlender, A. Boucher, V. Bourrier, S. Bovay, C. H. Broeg, D. Brousseau, A. Cabral, C. Cadieux, A. Carmona, Z. Challita, B. Chazelas, J. M. Coelho, M. Cointepas, A. R. Costa Silva, L. Coulombe, E. A. S. Cristo, A. Darveau-Bernier, L. Dauplaise, R. de Lima Gomes, D. O. Fontinele, Y. G. C. Frensch, F. Genest, L. Genolet, F. Gracia Témich, O. Hernandez, H. J. Hoeijmakers, N. Hubin, R. Jayawardhana, H. U. Käufl, D. Kerley, J. Kolb, V. Krishnamurthy, B. Kung, P. Lamontagne, O. Lim, G. Lo Curto, J. Luis Rasilla, A. M. Martins, J. Matthews, J. Mayer, Y. S. Messias, S. Metchev, D. Mounzer, N. Nari, A. Osborn, M. Ouellet, L. Parc, L. Pasquini, C. Peroux, C. Piaulet-Ghorayeb, E. Pompei, A.-S. Poulin-Girard, V. Reshetov, J. Saint-Antoine, M. Sarajlic, R. Schnell, A. Segovia, J. V. Seidel, A. Silber, P. Sinclair, M. Sordet, D. Sosnowska, A. Srivastava, A. K. Stefanov, M. A. Teixeira, S. Thibault, P. Vallée, T. Vandal, V. Vaulato, B. Wehbe, D. Weisserman, I. Wevers, F. Wildi, V. Yariv, G. Zins

Abstract
The intense stellar irradiation of ultra-hot Jupiters results in some of the most extreme atmospheric environments in the planetary regime. On their daysides, temperatures can be sufficiently high for key atmospheric constituents to thermally dissociate into simpler molecular species and atoms. This dissociation drastically changes the atmospheric opacities and, in turn, critically alters the temperature structure, atmospheric dynamics, and day-night heat transport. To date, however, simultaneous detections of the dissociating species and their thermally dissociation products in exoplanet atmospheres have remained rare. In this work we present the simultaneous detections of H₂O and its thermally dissociation product OH on the dayside of the ultra-hot Jupiter WASP-121 b based on high-resolution emission spectroscopy with the recently commissioned Near InfraRed Planet Searcher (NIRPS). We retrieved a photospheric abundance ratio of log₁₀(OH/H₂O) = −0.15 ± 0.20, indicating that there is about as much OH as H₂O at photospheric pressures, which confirms predictions from chemical equilibrium models. We compared the dissociation on WASP-121 b with other ultra-hot Jupiters and show that a trend in agreement with equilibrium models arises. We also discuss an apparent velocity shift of 4.79_−0.97^+0.93 km s⁻¹ in the H₂O signal, which is not reproduced by current global circulation models. Finally, in addition to H₂O and OH, the NIRPS data reveal evidence of Fe and Mg, from which we inferred a Fe/Mg ratio consistent with the solar and host star ratios. Our results demonstrate that NIRPS can be an excellent instrument to obtain simultaneous measurements of refractory and volatile molecular species, thus paving the way for many future studies on the atmospheric composition, chemistry, and the formation history of close-in exoplanets.

Keywords
atmospheric effects / instrumentation: spectrographs / techniques: spectroscopic / planets and satellites: atmospheres / planets and satellites: gaseous planets

Astronomy & Astrophysics
Volume 701, Article Number A276, Number of pages 21
2025 September

>> ADS>> DOI