H- and Dissociation in Ultra-hot Jupiters: A Retrieval Case Study of WASP-18b

Abstract

Atmospheres of a number of ultra-hot Jupiters (UHJs) with temperatures $\gtrsim$2000~K have been observed recently. Many of these planets show largely featureless thermal spectra in the near-infrared observed with the HST WFC3 spectrograph (1.1-1.7~$\mu$m) even though this spectral range contains strong H${}_2$O opacity. Recent works have proposed the possibility of H- opacity masking the H${}_2$O feature and/or thermal dissociation of H${}_2$O causing its apparent depletion at the high temperatures of UHJs. In this work we test these hypotheses using observations of the exoplanet WASP-18b as a case study. We report detailed atmospheric retrievals of the planet using the HyDRA retrieval code, extended to include the effects of H- opacity and thermal dissociation. We report constraints on the H${}_2$O, CO and H- abundances as well as the pressure-temperature profile of the dayside atmosphere for retrievals with and without H-/dissociation for each dataset. We find that the H${}_2$O and H- abundances are relatively unconstrained given the featureless WFC3 spectra. We do not conclusively detect H- in the planet contrary to previous studies which used equilibrium models to infer its presence. The constraint on the CO abundance depends on the combination of WFC3 and Spitzer data, ranging from solar to super-solar CO values. We additionally see signs of a thermal inversion from two of the datasets. Our study demonstrates the potential of atmospheric retrievals of UHJs including the effects of H- and thermal dissociation of molecules.

19 pages, 10 figures, accepted for publication in The Astronomical Journal

References in corpus (14)

• On the radiative equilibrium of irradiated planetary atmospheres
• A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b
• Can TiO Explain Thermal Inversions in the Upper Atmospheres of Irradiated Giant Planets?
• An ultrahot gas-giant exoplanet with a stratosphere
• Synthetic Spectra and Colors of Young Giant Planet Atmospheres: Effects of Initial Conditions and Atmospheric Metallicity
• ExoMol line lists XVIII. The high temperature spectrum of VO
• H2O abundances in the atmospheres of three hot Jupiters
• GENESIS: New Self-Consistent Models of Exoplanetary Spectra
• Climate of an Ultra hot Jupiter: Spectroscopic phase curve of WASP-18b with HST/WFC3
• Retrieval of Exoplanet Emission Spectra with HyDRA
• Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b
• New Avenues for Thermal Inversions in hot Jupiters
• An emission spectrum for WASP-121b measured across the 0.8-1.1 micron wavelength range using the Hubble Space Telescope
• The carbon-to-oxygen ratio: implications for the spectra of hydrogen-dominated exoplanet atmospheres

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