2019MNRAS.488.2222M

WASP-121b is a transiting gas giant exoplanet orbiting close to its Roche limit, with an inflated radius nearly double that of Jupiter and a dayside temperature comparable to a late M dwarf photosphere. Secondary eclipse observations covering the 1.1-1.6 µm wavelength range have revealed an atmospheric thermal inversion on the dayside hemisphere, likely caused by high-altitude absorption at optical wavelengths. Here we present secondary eclipse observations made with the Hubble Space Telescope Wide Field Camera 3 spectrograph that extend the wavelength coverage from 1.1 µm down to 0.8 µm. To determine the atmospheric properties from the measured eclipse spectrum, we performed a retrieval analysis assuming chemical equilibrium, with the effects of thermal dissociation and ionization included. Our best-fitting model provides a good fit to the data with reduced χ^2_ν=1.04. The data diverge from a blackbody spectrum and instead exhibit emission due to H^– shortward of 1.1 µm. The best-fitting model does not reproduce a previously reported bump in the spectrum at 1.25µm, possibly indicating this feature is a statistical fluctuation in the data rather than a VO emission band as had been tentatively suggested. We estimate an atmospheric metallicity of [ M/ H]= 1.09_–0.69^+0.57, and fit for the carbon and oxygen abundances separately, obtaining [ C/ H]= -0.29_–0.48^+0.61 and [ O/ H]= 0.18_–0.60^+0.64. The corresponding carbon-to-oxygen ratio is C/O = 0.49_–0.37^+0.65, which encompasses the solar value of 0.54, but has a large uncertainty.