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

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, H 2 O, 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.