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 mu m 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 mu m down to 0.8 mu m. 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 chi(2)(nu) = 1.04. The data diverge from a blackbody spectrum and instead exhibit emission due to H- shortward of 1.1 mu m. The best-fitting model does not reproduce a previously reported bump in the spectrum at 1.25 mu m, 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.NASA [NAS 5-26555]; Space Telescope Science Institute; Leverhulme Trust; University of Exeter PhD Studentship - UK Science and Technology Facilities Council (STFC) studentship; STFC Consolidated Grant [ST/R000395/1]; European Research Council;[ATMO 757858]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu