Hydride ion continuum hides absorption signatures in the NIRPS near-infrared transmission spectrum of the ultra-hot gas giant WASP-189b^★

¹ Observatoire de Genève, Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Institut Trottier de recherche sur les exoplanètes, Département de Physique, Université de Montréal, Montréal, Québec, Canada
³ Centre Vie dans l’Univers, Faculté des sciences de l’Université de Genève, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland
⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
⁶ Observatoire du Mont-Mégantic, Québec, Canada
⁷ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁸ Department of Physics, University of Toronto, Toronto, ON M5S 3H4, Canada
⁹ Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário, Natal, RN 59072-970, Brazil
¹⁰ Department of Physics & Astronomy, McMaster University, 1280 Main St W, Hamilton, ON L8S 4L8, Canada
¹¹ Department of Physics, McGill University, 3600 rue University, Montréal, QC H3A 2T8, Canada
¹² Department of Earth & Planetary Sciences, McGill University, 3450 rue University, Montréal, QC H3A 0E8, Canada
¹³ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹⁴ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁵ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁶ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁷ Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain
¹⁸ Bishop’s Univeristy, Dept of Physics and Astronomy, Johnson-104E, 2600 College Street, Sherbrooke, QC J1M 1Z7, Canada
¹⁹ Department of Physics and Space Science, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, ON, Canada
²⁰ European Southern Observatory (ESO), Av. Alonso de Cordova 3107, Casilla 19001, Santiago de Chile, Chile
²¹ Planétarium de Montréal, Espace pour la Vie, 4801 av. Pierre-de Coubertin, Montréal, Québec, Canada
²² Light Bridges S.L., Observatorio del Teide, Carretera del Observatorio, s/n Guimar, 38500 Tenerife, Canarias, Spain
²³ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, Campus UAB, Cerdanyola del Valles 08193, Spain
²⁴ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain

^★★ Corresponding author: valentina.vaulato@unige.ch

Received: 12 November 2024
Accepted: 3 March 2025

Abstract

Context. Ultra-hot Jupiters showcase one-of-a-kind extreme atmospheric conditions, including the dissociation of molecules into atomic species, ionisation, and significant day-to-night temperature contrasts. The proximity to their host stars exposes ultra-hot Jupiters to intense stellar irradiation, enabling high temperatures that drive noteworthy contributions to the overall opacity by hydride ions (H⁻), potentially obscuring features of metals in the near-infrared transmission spectrum.

Aims. This work aims to detect atomic, ionic, and molecular species in the atmosphere of WASP-189b (H, He, Fe, Ti, V, Mn, Na, Mg, Ca, Cr, Ni, Y, Ba, Sc, Fe⁺, Ti⁺, TiO, H₂O, CO, and OH). A focus is placed on (i) understanding the role of H⁻ as a source of absorption continuum opacity, and (ii) retrieving the relative hydride-to-Fe abundance using combined optical and near-infrared data.

Methods. We present two transits of WASP-189b gathered simultaneously in the optical with HARPS and in the near-infrared with NIRPS, supported by photometric light curves from EulerCam and ExTrA. Transmission spectra were analysed via cross-correlation to detect a planet’s absorption features and to increase the signal-to-noise ratio of potential detections. Additionally, atmospheric retrievals quantified relative abundances by fitting the overall metallicity, and abundance proxies for TiO, H⁻, and e⁻.

Results. Only atomic iron is detected in HARPS data (S/N~5.5). However, no Fe is detected at near-infrared wavelengths, likely due to the H⁻ continuum dampening. Atmospheric retrievals on HARPS only and HARPS+NIRPS combined suggest that the hydride-to-Fe ratio exceeds equilibrium model predictions by ~0.5 dex, hinting at a strong ionisation rate for hydrogen atoms. Including NIRPS data helps to constrain the H⁻ abundance, as well as set an upper limit on the free electron density, which is unconstrained from the HARPS-only retrieval. These results emphasise the impact of H⁻ as a non-negligible source of continuum absorption opacity impeding the detection of planetary absorption features in the near-infrared transmission spectrum of WASP-189b.