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

TL;DR

Ultra-hot Jupiters, such as WASP-121 b, host dayside atmospheres hot enough to thermally dissociate water into OH, reshaping the thermal structure and chemistry. Using high-resolution emission spectroscopy with NIRPS, the study simultaneously detects H$_2$O and OH, and retrieves abundances for H$_2$O, OH, Fe, and Mg along with atmospheric temperature structure and dynamical velocities. The results yield a OH/H$_2$O ratio of $\ $ $\log_{10}(\mathrm{OH}/\mathrm{H_2O}) = -0.15 \pm 0.20$, consistent with chemical equilibrium at ~1 mbar and indicating substantial but not complete water dissociation; a notable redshift in H$_2$O relative to other species is found, not fully reproduced by current GCMs. The Fe/Mg abundance ratio is consistent with solar and stellar values, and the retrieved K$_p$ aligns with expectations when planetary rotation is included, validating NIRPS as a powerful instrument for probing both volatile and refractory species and informing exoplanetary formation histories.

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.

Figures (14)

• Figure 1: Summary of the observations of WASP-121 b. The top panel is the airmass as a function of phase for all observations. The bottom panel shows the S/N per pixel throughout the observations. The grey shaded area centred around phase 0.5 represents the phases where WASP-121 b is eclipsed by its host star. Of the six observations, three of them were obtained during the pre-eclipse phases (phases $<$ 0.5), while the other three were of the post-eclipse phases (phases $>$ 0.5).
• Figure 2: Cross-correlation function of each observation with an OH model (black-and-white colour map), along with the expected WASP-121 b velocities (black dashed line) compared to where tellurics are in velocity space (blue dotted line). The cross-correlation maps are shown in the BERV-corrected frame. In none of our observation nights do the planet velocities overlap with where the telluric are, lowering the risk that any remaining telluric residuals affect our results.
• Figure 3: Cross-sections of H$_2$O, OH, Fe and Mg in the wavelength range of NIRPS. The cross-correlations are calculated at a pressure of 1 mbar and a temperature of 2500 K. H$_2$O contains a forest of lines throughout the entire wavelength range shown here. OH is prominent at wavelengths longer than 1.4 $\mu$m but still has strong lines around 1 $\mu$m. Fe features are mostly present in the shorter wavelengths with its most important lines below 1.2$\,\mu$m. Mg has few lines overall.
• Figure 4: Cross-correlation signal-to-noise maps of H$_2$O, CO, Fe, and Mg in the atmosphere of WASP-121 b. The dashed vertical and horizontal white lines represent the stellar $V_{\mathrm{sys}}$artigau_line-by-line_2022 and $K_p$bourrier_hot_2020 of WASP-121 b, respectively. Tentative signals of H$_2$O, OH, Fe and Mg can be seen near the expected orbital position of WASP-121 b.
• Figure 5: Results from the atmospheric retrieval of WASP-121 b. The left corner plot shows the posteriors of the retrieved parameters, with the marginalised 1D posteriors on the diagonal and the 2D posteriors on the off-diagonals. The contours on the 2D posteriors represent the 1, 2 and 3 $\sigma$ uncertainties. The first 4 parameters are the base-10 logarithm of volume mixing ratios of the four detected species: H$_2$O, OH, Fe and Mg. The 1D posteriors for the retrieved abundances show relatively broad distributions. However, 2D posteriors between abundances depict a strong positive correlation, indicating that the abundance ratios are better constrained. logH$^{-}$ and loge$^{-}$ are the volume mixing ratio of the H$^{-}$ radical and of electrons, respectively, used as broad band opacities. Only a weak upper bound is obtained on logH$^{-}$ while the posterior abundance of loge$^{-}$ spans the full prior. The grey lines on the $K_p$ and $V_{\mathrm{sys}}$ parameters are the expected orbital velocities of the planet bourrier_hot_2020brown_gaia_2018. Both orbital parameters are consistent with expectations. $\Delta V_{\mathrm{sys, H_2O}}$ is the additional $V_{\mathrm{sys}}$ shift added to H$_2$O. The posterior is constrained to a value above 0, indicating that the retrieval also finds the shift seen in the cross-correlation maps (Fig. \ref{['fig:kpvsys']}). The top right plot is the retrieved vertical temperature profile, with the shaded areas representing the 1 and 2 $\sigma$ uncertainties. The profile has a temperature inversion, which is expected for UHJs.