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

• DOI: 10.1093/mnras/stu2279
• Publication Date: 2014-11-28T10:35:07Z
• Authors: D. K. Sing; H. R. Wakeford; A. P. Showman; N. Nikolov; J. J. Fortney; A. S. Burrows; G. E. Ballester; D. Deming; S. Aigrain; J.-M. Désert; N. P. Gibson; G. W. Henry; H. Knutson; A. Lecavelier des...; F. Pont; A. Vidal-Madjar; M. W. Williamson; P. A. Wilson
• Keywords: 330; FOS: Physical sciences; Astronomy & Astrophysics; stars: individual: WASP-31; [SDU.ASTR] Sciences of the Universe [physics]/Astrophysics [astro-ph]; 530; 01 natural sciences; 7. Clean energy; individual: WASP-31 [stars]; planets and satellites: individual: WASP-31b; [SDU] Sciences of the Universe [physics]; 0103 physical sciences; individual: WASP-31b [planets and satellites]; planetary systems; planets and satellites: atmospheres; Earth and Planetary Astrophysics (astro-ph.EP); atmospheres [planets and satellites]; 520; 13. Climate action; astro-ph.EP; spectroscopic [techniques]; Astronomical and Space Sciences; techniques: spectroscopic; Astrophysics - Earth and Planetary Astrophysics

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 $��$m at a resolution $R\sim$70, which we combine with Spitzer photometry to cover the full-optical to IR. The spectrum is dominated by a cloud-deck with a flat transmission spectrum which is apparent at wavelengths $>0.52��$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-$��$ confidence level. Broadened alkali wings are not detected, indicating pressures below $\sim$10 mbar. The lack of Na and strong K is the first indication of a sub-solar Na/K abundance ratio in a planetary atmosphere (ln[Na/K]$=-3.3\pm2.8$), which could potentially be explained by Na condensation on the planet's night side, or primordial abundance variations. A strong Rayleigh scattering signature is detected at short wavelengths, with a 4-$��$ 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.<br/>18 pages, 13 figures, 3 tables. Accepted for publication in MNRAS<br/>