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WASP-121b is a transiting gas giant exoplanet orbiting close to its Rochelimit, with an inflated radius nearly double that of Jupiter and a daysidetemperature comparable to a late M dwarf photosphere. Secondary eclipseobservations covering the 1.1-1.6 micron wavelength range have revealed anatmospheric thermal inversion on the dayside hemisphere, likely caused by highaltitude absorption at optical wavelengths. Here we present secondary eclipseobservations made with the Hubble Space Telescope Wide Field Camera 3spectrograph that extend the wavelength coverage from 1.1 micron down to 0.8micron. To determine the atmospheric properties from the measured eclipsespectrum, we performed a retrieval analysis assuming chemical equilibrium, withthe effects of thermal dissociation and ionization included. Our best-fit modelprovides a good fit to the data with reduced chi^2=1.04. The data diverge froma blackbody spectrum and instead exhibit emission due to H- shortward of 1.1micron. The best-fit model does not reproduce a previously reported bump in thespectrum at 1.25 micron, possibly indicating this feature is a statisticalfluctuation in the data rather than a VO emission band as had been tentativelysuggested. 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 correspondingcarbon-to-oxygen ratio is C/O=0.49(-0.37,+0.65), which encompasses the solarvalue of 0.54, but has a large uncertainty.

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