Confirmation of water emission in the dayside spectrum of the ultrahot Jupiter WASP-121b

Spake, Jessica J.; Barstow, Joanna K.; Lewis, Nikole K.; Mayne, Nathan J.; Wakeford, Hannah R.; Kataria, Tiffany; Sing, David K. and Mikal-Evans, Thomas (2020). Confirmation of water emission in the dayside spectrum of the ultrahot Jupiter WASP-121b. Monthly Notices of the Royal Astronomical Society, 496(2) pp. 1638–1644.



We present four new secondary eclipse observations for the ultrahot Jupiter WASP-121b acquired using the Hubble Space Telescope Wide Field Camera 3. The eclipse depth is measured to a median precision of 60 ppm across 28 spectroscopic channels spanning the 1.12 - 1.64μm wavelength range. This is a considerable improvement to the 90 ppm precision we achieved previously for a single eclipse observation using the same observing set-up. Combining these data with those reported at other wavelengths, a blackbody spectrum for WASP-121b is ruled out at >6σ confidence and we confirm the interpretation of previous retrieval analyses that found the data are best explained by a dayside thermal inversion. The updated spectrum clearly resolves the water emission band at 1.3 - 1.6μm , with higher signal-to-noise than before. It also fails to reproduce a bump in the spectrum at 1.25μm derived from the first eclipse observation, which had tentatively been attributed to VO emission. We conclude that the latter was either a statistical fluctuation or a systematic artefact specific to the first eclipse data set.

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