This is the author accepted manuscript. The final version is available from American Astronomical Society / IOP Publishing via the DOI in this record.We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). Across the 0.47-1 micron 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 micron). Wavelength-dependent variations in the opacity rule out a gray cloud deck at a confidence level of 3.8-sigma and may instead be explained by VO spectral bands. We find a cloud-free model assuming chemical equilibrium for a temperature of 1500K and 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 micron in wavelength. If this feature is caused by Rayleigh scattering due to uniformly-distributed aerosols, it would imply an unphysically high temperature of 6810+/-1530K. 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.Support for program GO-14767 was provided by
NASA through a grant from the Space Telescope Science Institute, which is operated
by the Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS 5-26555. T.M.E., D.K.S., and N.N. acknowledge funding from the European
Research Council under the European Unions Seventh Framework Programme
(FP7/2007-2013)/ERC grant agreement no. 336792. G.W.H. and M.H.W. acknowledge
support from Tennessee State University and the State of Tennessee through its
Centers of Excellence program. J.S.F. acknowledges funding by the Spanish MINECO
grant AYA2016-79425-C3-2-P. J.K.B. is supported by a Royal Astronomical Society
Research Fellowship. This work has been carried out in the frame of the National
Centre for Competence in Research PlanetS supported by the Swiss National Science
Foundation (SNSF). V.B. and D.E. have received funding from the European
Research Council (ERC) under the European Unions Horizon 2020 research and innovation
programme (project Four Aces; grant agreement no. 724427)
