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We report Gemini-North GMOS observations of the inflated hot JupiterHAT-P-32b during two primary transits. We simultaneously observed twocomparison stars and used differential spectro-photometry to producemulti-wavelength light curves. 'White' light curves and 29 'spectral' lightcurves were extracted for each transit and analysed to refine the systemparameters and produce transmission spectra from 520-930nm in ~14nm bins. Thelight curves contain time-varying white noise as well as time-correlated noise,and we used a Gaussian process model to fit this complex noise model. Commonmode corrections derived from the white light curve fits were applied to thespectral light curves which significantly improved our precision, reachingtypical uncertainties in the transit depth of ~2x10^-4, corresponding to abouthalf a pressure scale height. The low resolution transmission spectra areconsistent with a featureless model, and we can confidently rule out broadfeatures larger than about one scale height. The absence of Na/K wings orprominent TiO/VO features is most easily explained by grey absorption fromclouds in the upper atmosphere, masking the spectral features. However, wecannot confidently rule out clear atmosphere models with low abundances (~10^-3solar) of TiO, VO or even metal hydrides masking the Na and K wings. A smallerscale height or ionisation could also contribute to muted spectral features,but alone are unable to to account for the absence of features reported here.

The optical transmission spectrum of the hot Jupiter HAT-P-32b: clouds explain the absence of broad spectral features?