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An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the Hubble Space Telescope

Antonio García Muñoz Alain Lecavelier Des Etangs Eric Hébrard 1 Thomas Evans David Sing 2 Jayesh Goyal 3 Nikolay Nikolov 4 Mark Marley 5 Kevin Zahnle 6 Gregory Henry Joanna Barstow 7 Munazza Alam Jorge Sanz-Forcada 8 Tiffany Kataria 9 Nikole Lewis 10 Panayotis Lavvas 11 Gilda Ballester 11 Lotfi Ben-Jaffel 2 Sarah Blumenthal Vincent Bourrier 12 Benjamin Drummond 13 Antonio Muñoz Mercedes López-Morales 14 Pascal Tremblin 15 David Ehrenreich 12 Hannah Wakeford 16 Lars Buchhave 17 Alain Lecavelier Des Etangs Ric Hébrard Michael Williamson
Abstract : We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Across the 0.47-1 mm wavelength range, the data imply an atmospheric opacity comparable to-and in some spectroscopic channels exceeding-that previously measured at near-infrared wavelengths (1.15-1.65 mm). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.7σ and may instead be explained by VO spectral bands. We find a cloud-free model assuming chemical equilibrium for a temperature of 1500 K and a metal enrichment of 10-30× solar matches these data well. Using a free-chemistry retrieval analysis, we estimate a VO abundance of-+ 6.6 0.3 0.2 dex. We find no evidence for TiO and place a 3σ upper limit of −7.9 dex on its abundance, suggesting TiO may have condensed from the gas phase at the day-night limb. The opacity rises steeply at the shortest wavelengths, increasing by approximately five pressure scale heights from 0.47 to 0.3 mm in wavelength. If this feature is caused by Rayleigh scattering due to uniformly distributed aerosols, it would imply an unphysically high temperature of 6810±1530 K. One alternative explanation for the short-wavelength rise is absorption due to SH (mercapto radical), which has been predicted as an important product of non-equilibrium chemistry in hot Jupiter atmospheres. Irrespective of the identity of the NUV absorber, it likely captures a significant amount of incident stellar radiation at low pressures, thus playing a significant role in the overall energy budget, thermal structure, and circulation of the atmosphere.
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Submitted on : Wednesday, December 16, 2020 - 5:24:55 PM
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Antonio García Muñoz, Alain Lecavelier Des Etangs, Eric Hébrard, Thomas Evans, David Sing, et al.. An Optical Transmission Spectrum for the Ultra-hot Jupiter WASP-121b Measured with the Hubble Space Telescope. Astronomical Journal, American Astronomical Society, 2018, 156 (6), pp.283. ⟨10.3847/1538-3881/aaebff⟩. ⟨hal-02381621⟩

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