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arXiv · 2508.06626

Quantifying thermal water dissociation in the dayside photosphere of WASP-121 b using NIRPS

Luc Bazinet·Romain Allart·Björn Benneke·Stefan Pelletier·Joost P. Wardenier·Neil J. Cook·Thierry Forveille·Louise D. Nielsen·Khaled Al Moulla·Étienne Artigau·Frédérique Baron·Susana C. C. Barros·Xavier Bonfils·François Bouchy·Marta Bryan·Bruno L. Canto Martins·Ryan Cloutier·Nicolas B. Cowan·Daniel Brito de Freitas·Jose Renan De Medeiros·Xavier Delfosse·René Doyon·Xavier Dumusque·David Ehrenreich·Jonay I. González Hernández·David Lafrenière·Izan de Castro Leão·Christophe Lovis·Lison Malo·Claudio Melo·Lucile Mignon·Christoph Mordasini·Francesco Pepe·Rafael Rebolo·Jason Rowe·Nuno C. Santos·Damien Ségransan·Alejandro Suárez Mascareño·Stéphane Udry·Diana Valencia·Gregg Wade·Manuel Abreu·José L. A. Aguiar·Guillaume Allain·Tomy Arial·Hugues Auger·Nicolas Blind·David Bohlender·Anne Boucher·Vincent Bourrier·Sébastien Bovay·Christopher Broeg·Denis Brousseau·Alexandre Cabral·Charles Cadieux·Andres Carmona·Zalpha Challita·Bruno Chazelas·João Coelho·Marion Cointepas·Ana Rita Costa Silva·Louis-Philippe Coulombe·Eduardo Cristo·Antoine Darveau-Bernier·Laurie Dauplaise·Roseane de Lima Gomes·Dasaev O. Fontinele·Yolanda G. C. Frensch·Frédéric Genest·Ludovic Genolet·Félix Gracia Témich·Olivier Hernandez·H. Jens Hoeijmakers·Norbert Hubin·Ray Jayawardhana·Hans-Ulrich Käufl·Dan Kerley·Johann Kolb·Vigneshwaran Krishnamurthy·Benjamin Kung·Pierrot Lamontagne·Olivia Lim·Gaspare Lo Curto·José Luis Rasilla·Allan M. Martins·Jaymie Matthews·Jean-Sébastien Mayer·Yuri S. Messias·Stan Metchev·Dany Mounzer·Nicola Nari·Ares Osborn·Mathieu Ouellet·Léna Parc·Luca Pasquini·Céline Peroux·Caroline Piaulet-Ghorayeb·Emanuela Pompei·Anne-Sophie Poulin-Girard·Vladimir Reshetov
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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 this date, however, simultaneous detections of the dissociating species and their thermally dissociation products in exoplanet atmospheres have remained rare. Here we present the simultaneous detections of H$_2$O 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 retrieve a photospheric abundance ratio of log$_{10}$(OH/H$_2$O) $= -0.15\pm{0.20}$ indicating that there is about as much OH as H$_2$O at photospheric pressures, which confirms predictions from chemical equilibrium models. We compare 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^{+0.93}_{-0.97} $km s$^{-1}$ in the H$_2$O signal, which is not reproduced by current global circulation models. Finally, in addition to H$_2$O and OH, the NIRPS data reveal evidence of Fe and Mg, from which we infer 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, paving the way for many future studies on the atmospheric composition, chemistry, and the formation history of close-in exoplanets.

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Luc Bazinet, Romain Allart, Björn Benneke, Stefan Pelletier, Joost P. Wardenier, Neil J. Cook, Thierry Forveille, Louise D. Nielsen, Khaled Al Moulla, Étienne Artigau, Frédérique Baron, Susana C. C. Barros, Xavier Bonfils, François Bouchy, Marta Bryan, Bruno L. Canto Martins, Ryan Cloutier, Nicolas B. Cowan, Daniel Brito de Freitas, Jose Renan De Medeiros, Xavier Delfosse, René Doyon, Xavier Dumusque, David Ehrenreich, Jonay I. González Hernández, David Lafrenière, Izan de Castro Leão, Christophe Lovis, Lison Malo, Claudio Melo, Lucile Mignon, Christoph Mordasini, Francesco Pepe, Rafael Rebolo, Jason Rowe, Nuno C. Santos, Damien Ségransan, Alejandro Suárez Mascareño, Stéphane Udry, Diana Valencia, Gregg Wade, Manuel Abreu, José L. A. Aguiar, Guillaume Allain, Tomy Arial, Hugues Auger, Nicolas Blind, David Bohlender, Anne Boucher, Vincent Bourrier, Sébastien Bovay, Christopher Broeg, Denis Brousseau, Alexandre Cabral, Charles Cadieux, Andres Carmona, Zalpha Challita, Bruno Chazelas, João Coelho, Marion Cointepas, Ana Rita Costa Silva, Louis-Philippe Coulombe, Eduardo Cristo, Antoine Darveau-Bernier, Laurie Dauplaise, Roseane de Lima Gomes, Dasaev O. Fontinele, Yolanda G. C. Frensch, Frédéric Genest, Ludovic Genolet, Félix Gracia Témich, Olivier Hernandez, H. Jens Hoeijmakers, Norbert Hubin, Ray Jayawardhana, Hans-Ulrich Käufl, Dan Kerley, Johann Kolb, Vigneshwaran Krishnamurthy, Benjamin Kung, Pierrot Lamontagne, Olivia Lim, Gaspare Lo Curto, José Luis Rasilla, Allan M. Martins, Jaymie Matthews, Jean-Sébastien Mayer, Yuri S. Messias, Stan Metchev, Dany Mounzer, Nicola Nari, Ares Osborn, Mathieu Ouellet, Léna Parc, Luca Pasquini, Céline Peroux, Caroline Piaulet-Ghorayeb, Emanuela Pompei, Anne-Sophie Poulin-Girard, Vladimir Reshetov. 2025-08-08. Quantifying thermal water dissociation in the dayside photosphere of WASP-121 b using NIRPS. https://doi.org/10.1051/0004-6361%2F202553724

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