HST hot-Jupiter transmission spectral survey: detection of potassium in WASP-31b along with a cloud deck and Rayleigh scattering

Abstract

We present Hubble Space Telescope optical and near-IR transmission spectra of\nthe transiting hot-Jupiter WASP-31b. The spectrum covers 0.3-1.7 $\\mu$m at a\nresolution $R\\sim$70, which we combine with Spitzer photometry to cover the\nfull-optical to IR. The spectrum is dominated by a cloud-deck with a flat\ntransmission spectrum which is apparent at wavelengths $>0.52\\mu$m. The cloud\ndeck is present at high altitudes and low pressures, as it covers the majority\nof the expected optical Na line and near-IR H$_2$O features. While Na I\nabsorption is not clearly identified, the resulting spectrum does show a very\nstrong potassium feature detected at the 4.2-$\\sigma$ confidence level.\nBroadened alkali wings are not detected, indicating pressures below $\\sim$10\nmbar. The lack of Na and strong K is the first indication of a sub-solar Na/K\nabundance ratio in a planetary atmosphere (ln[Na/K]$=-3.3\\pm2.8$), which could\npotentially be explained by Na condensation on the planet's night side, or\nprimordial abundance variations. A strong Rayleigh scattering signature is\ndetected at short wavelengths, with a 4-$\\sigma$ significant slope. Two\ndistinct aerosol size populations can explain the spectra, with a smaller\nsub-micron size grain population reaching high altitudes producing a blue\nRayleigh scattering signature on top of a larger, lower-lying population\nresponsible for the flat cloud deck at longer wavelengths. We estimate that the\natmospheric circulation is sufficiently strong to mix micron size particles\nupward to the required 1-10 mbar pressures, necessary to explain the cloud\ndeck. These results further confirm the importance of clouds in hot-Jupiters,\nwhich can potentially dominate the overall spectra and may alter the abundances\nof key gaseous species.\n