Journal
ASTRONOMICAL JOURNAL
Volume 156, Issue 6, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.3847/1538-3881/aaebff
Keywords
methods: observational; planets and satellites: atmospheres; planets and satellites: gaseous planets
Categories
Funding
- NASA through Space Telescope Science Institute [GO-14767]
- NASA [NAS 5-26555]
- European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC [336792]
- Leverhulme Trust Research Project Grant
- University of Exeter PhD Studentship
- Tennessee State University
- State of Tennessee through its Centers of Excellence program
- Spanish MINECO grant [AYA2016-79425-C3-2-P]
- Royal Astronomical Society Research Fellowship
- CNES (France) under project PACES
- Swiss National Science Foundation (SNSF)
- European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (project FOUR ACES) [724427]
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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 mu m 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 mu m). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.7 sigma 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 10-30x 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 3 sigma 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 mu m 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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