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We present the complete optical transmission spectrum of the hot JupiterWASP-4b from 440-940 nm at R ~ 400-1500 obtained with the Gemini Multi-ObjectSpectrometers (GMOS); this is the first result from a comparativeexoplanetology survey program of close-in gas giants conducted with GMOS.WASP-4b has an equilibrium temperature of 1700 K and is favorable to study intransmission due to a large scale height (370 km). We derive the transmissionspectrum of WASP-4b using 4 transits observed with the MOS technique. Wedemonstrate repeatable results across multiple epochs with GMOS, and derive acombined transmission spectrum at a precision about twice above photon noise,which is roughly equal to to one atmospheric scale height. The transmissionspectrum is well fitted with a uniform opacity as a function of wavelength. Theuniform opacity and absence of a Rayleigh slope from molecular hydrogen suggestthat the atmosphere is dominated by clouds with condensate grain size of ~1 um.This result is consistent with previous observations of hot Jupiters sinceclouds have been seen in planets with similar equilibrium temperatures toWASP-4b. We describe a custom pipeline that we have written to reduce GMOStime-series data of exoplanet transits, and present a thorough analysis of thedominant noise sources in GMOS, which primarily consist of wavelength- andtime- dependent displacements of the spectra on the detector, mainly due to alack of atmospheric dispersion correction.

Gemini/GMOS Transmission Spectral Survey: Complete Optical Transmission Spectrum of the hot Jupiter WASP-4b