arXiv · 2507.21229
Hydride ion continuum hides absorption signatures in the NIRPS near-infrared transmission spectrum of the ultra-hot gas giant WASP-189b
Abstract
Ultra-hot Jupiters showcase extreme atmospheric conditions, including molecular dissociation, ionisation, and significant day-to-night temperature contrasts. Their close proximity to host stars subjects them to intense stellar irradiation, driving high temperatures where hydride ions (H$^-$) significantly contribute to opacity, potentially obscuring metal features in near-infrared transmission spectra. We investigate the atmosphere of WASP-189b, targeting atomic, ionic, and molecular species (H, He, Fe, Ti, V, Mn, Na, Mg, Ca, Cr, Ni, Y, Ba, Sc, Fe$^+$, Ti$^+$, TiO, H$_2$O, CO, and OH), focusing on (i) the role of H$^-$ as a source of continuum opacity, and (ii) the relative hydride-to-Fe abundance using joint optical and near-infrared data. We present two transits of WASP-189b gathered simultaneously in the optical with HARPS and near-infrared with NIRPS, supported by photometric light curves from EulerCam and ExTrA. Transmission spectra were analysed via cross-correlation to detect absorption features and enhance the signal-to-noise ratio. Atmospheric retrievals quantified relative abundances by fitting overall metallicity and proxies for TiO, H$^-$, and e$^-$. Only atomic iron is detected in HARPS data (S/N ~5.5), but not in NIRPS, likely due to H$^-$ continuum dampening. Retrievals on HARPS-only and HARPS+NIRPS suggest the hydride-to-Fe ratio exceeds equilibrium predictions by about 0.5 dex, hinting at strong hydrogen ionisation. Including NIRPS data helps constrain H$^-$ abundance and set an upper limit on free electron density, unconstrained in HARPS-only data. These results emphasise H$^-$ as a significant continuum opacity source impeding detection of planetary absorption features in WASP-189b's near-infrared transmission spectrum.
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Valentina Vaulato, Stefan Pelletier, David Ehrenreich, Romain Allart, Eduardo Cristo, Michal Steiner, Xavier Dumusque, Hritam Chakraborty, Monika Lendl, Avidaan Srivastava, Étienne Artigau, Frédérique Baron, C. Susana Barros, Björn Benneke, Xavier Bonfils, François Bouchy, Marta Bryan, Bruno L. Canto Martins, Ryan Cloutier, Neil J. Cook, Nicolas B. Cowan, Jose Renan De Medeiros, Xavier Delfosse, René Doyon, I. Jonay González Hernández, David Lafrenière, Izan de Castro Leão, Christophe Lovis, Lison Malo, Claudio Melo, Lucile Mignon, Christoph Mordasini, Franceso Pepe, Rafael Rebolo, Jason Rowe, Nuno C. Santos, Damien Ségransan, Alejandro Suárez Mascareño, Stéphane Udry, Diana Valencia, Gregg Wade, Khaled Al Moulla, Luc Bazinet, Vincent Bourrier, Charles Cadieux, Yann Carteret, Ana Rita Costa Silva, Antoine Darveau-Bernier, Laurie Dauplaise, Roseane de Lima Gomes, Jean-Baptiste Delisle, Thierry Forveille, Yolanda Frensch, Jonathan Gagné, Frédéric Genest, João Gomes da Silva, Nolan Grieves, Melissa J. Hobson, Vigneshwaran Krishnamurthy, Alexandrine L'Heureux, Pierrot Lamontagne, Pierre Larue, Olivia Lim, Gaspare Lo Curto, Lina Messamah, Yury S. Messias, Leslie Moranta, Dany Mounzer, Nicola Nari, Ares Osborn, Léna Parc, Caroline Piaulet, Mykhaylo Plotnykov, Angelica Psaridi, Atanas K. Stefanov, Márcio A. Teixeira, Thomas Vandal, Joost P. Wardenier, Drew Weisserman, Vincent Yariv. 2025-07-28. Hydride ion continuum hides absorption signatures in the NIRPS near-infrared transmission spectrum of the ultra-hot gas giant WASP-189b. https://doi.org/10.1051/0004-6361%2F202452972
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