Phase-dependent chemistry of WASP-43 b revealed with a suite of one-, two-, and three-dimensional models

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Abstract
Context. Spectroscopic phase curves of tidally locked giant exoplanets provide us with a unique global picture of their atmospheric chemistry, as different regions of the planet rotate in and out of view. However, the spatially changing composition of tidally locked exoplanets – governed by stellar irradiation and atmospheric circulation – make phase-resolved observations exceptionally hard to interpret. Aims. Our goal is to investigate the chemistry of the hot Jupiter WASP-43 b in detail using theoretical models, considering the constraints of the James Webb Space Telescope’s MIRI phase curve. Methods. With a suite of pseudo-two-dimensional and three-dimensional photochemical models, we simulated the composition of WASP-43 b in various configurations (different metallicities and wind speeds), and we compared them with atmospheric retrieval models. Additionally, we probed the robustness of photochemical predictions by comparing the results of four one-dimensional models that have equivalent setups. Results. We confirm that disequilibrium chemistry in our theoretical models reduces the methane concentration on the planet’s night side for wind jet speeds ≳500 m s −1 . Varying the metallicity in the models induces large changes in the CO 2 and SO 2 concentrations, with SO 2 producing mid-infrared absorption features in synthetic emission spectra of the night side at atmospheric metallicities ≳10× solar. Our models provide evidence for pole-to-equator circulation enhancing the CH 4 , NH 3 , and HCN abundances, which is nonetheless insufficient for detectable spectral features. Finally, we show that H 2 O, CO, and CO 2 are robustly modelled, but species affected by photochemistry (CH 4 , NH 3 , SO 2 , and HCN) are more sensitive to model-specific assumptions and pathways. Conclusions. We conclude that horizontal quenching is the prime mechanism that explains the non-detection of methane in the MIRI phase curve of WASP-43 b. This mechanism requires only moderate wind speeds and is operative at various thermal structures and atmospheric metallicities. Furthermore, coupled carbon–sulfur chemistry leads to an additional decrease in methane compared to previous models in the literature that did not contain sulfur chemistry. We do not favour a high metallicity as it would have led to observable SO 2 features in the MIRI spectra. Our study shows that phase-dependent photochemistry models are essential tools in the interpretation of hot-Jupiter phase curves, but benchmarking is needed to improve the accuracy of photochemical models in the future.
Funding Information
  • Nederlandse Organisatie voor Wetenschappelijk Onderzoek (614.001.601)
  • Nederlandse Organisatie voor Wetenschappelijk Onderzoek (VI.Veni.242.190)

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