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

D. K. Sing, H. R. Wakeford, A. P. Showman, N. Nikolov, J. J. Fortney, Adam S. Burrows, G. E. Ballester, D. Deming, S. Aigrain, J. M. Désert, N. P. Gibson, G. W. Henry, H. Knutson, A. Lecavelier Des Etangs, F. Pont, A. Vidal-Madjar, M. W. Williamson, P. A. Wilson