We present Hubble Space Telescope optical and near-IR transmission spectra of the transiting hot-Jupiter WASP-31b. The spectrum covers 0.3-1.7 $mu$m at a resolution $Rsim$70, which we combine with Spitzer photometry to cover the full-optical to IR. T
he spectrum is dominated by a cloud-deck with a flat transmission spectrum which is apparent at wavelengths $>0.52mu$m. The cloud deck is present at high altitudes and low pressures, as it covers the majority of the expected optical Na line and near-IR H$_2$O features. While Na I absorption is not clearly identified, the resulting spectrum does show a very strong potassium feature detected at the 4.2-$sigma$ confidence level. Broadened alkali wings are not detected, indicating pressures below $sim$10 mbar. The lack of Na and strong K is the first indication of a sub-solar Na/K abundance ratio in a planetary atmosphere (ln[Na/K]$=-3.3pm2.8$), which could potentially be explained by Na condensation on the planets night side, or primordial abundance variations. A strong Rayleigh scattering signature is detected at short wavelengths, with a 4-$sigma$ significant slope. Two distinct aerosol size populations can explain the spectra, with a smaller sub-micron size grain population reaching high altitudes producing a blue Rayleigh scattering signature on top of a larger, lower-lying population responsible for the flat cloud deck at longer wavelengths. We estimate that the atmospheric circulation is sufficiently strong to mix micron size particles upward to the required 1-10 mbar pressures, necessary to explain the cloud deck. These results further confirm the importance of clouds in hot-Jupiters, which can potentially dominate the overall spectra and may alter the abundances of key gaseous species.
We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting hot-Jupiter HAT-P-1b. We observed one transit with Wide Field Camera 3 using the G141 low-resolution grism to cover the wavelength range 1.087- 1.678 {mu}m. T
hese time series observations were taken with the newly available spatial scan mode that increases the duty cycle by nearly a factor of two, thus improving the resulting photometric precision of the data. We measure a planet-to-star radius ratio of Rp/R*=0.11709+/-0.00038 in the white light curve with the centre of transit occurring at 2456114.345+/-0.000133 (JD). We achieve S/N levels per exposure of 1840 (0.061%) at a resolution of {Deltalambda}=19.2nm (R~70) in the 1.1173 - 1.6549{mu}m spectral region, providing the precision necessary to probe the transmission spectrum of the planet at close to the resolution limit of the instrument. We compute the transmission spectrum using both single target and differential photometry with similar results. The resultant transmission spectrum shows a significant absorption above the 5-{sigma} level matching the 1.4{mu}m water absorption band. In solar composition models, the water absorption is sensitive to the ~1 mbar pressure levels at the terminator. The detected absorption agrees with that predicted by an 1000 K isothermal model, as well as with that predicted by a planetary-averaged temperature model.
