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 1.1–1.6μm 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 1.1μm down to 0.8μm. 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 χ2ν=1.04. The data diverge from a blackbody spectrum and instead exhibit emission due to H− shortward of 1.1μm. 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+0.57−0.69, and fit for the carbon and oxygen abundances separately, obtaining [C/H]=−0.29+0.61−0.48 and [O/H]=0.18+0.64−0.60. The corresponding carbon-to-oxygen ratio is C/O=0.49+0.65−0.37, which encompasses the solar value of 0.54, but has a large uncertainty.
Thomas Mikal-Evans, David K Sing, Jayesh M Goyal, Benjamin Drummond, Aarynn L Carter, Gregory W Henry, Hannah R Wakeford, Nikole K Lewis, Mark S Marley, Pascal Tremblin, Nikolay Nikolov, Tiffany Kataria, Drake Deming, Gilda E Ballester, An emission spectrum for WASP-121b measured across the 0.8–1.1 μm wavelength range using the Hubble Space Telescope, Monthly Notices of the Royal Astronomical Society, Volume 488, Issue 2, September 2019, Pages 2222–2234, https://doi.org/10.1093/mnras/stz1753