An emission spectrum for WASP-121b measured across the 0.8-1.1 μm wavelength range using the Hubble Space Telescope

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 1.25 μm, 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.