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

We present the complete optical transmission spectrum of the hot Jupiter WASP-4b from 440 to 940 nm at R ˜ 400-1500 obtained with theGemini Multi-Object Spectrometers (GMOS); this is the first result froma comparative exoplanetology survey program of close-in gas giantsconducted with GMOS. WASP-4b has an equilibrium temperature of 1700 Kand is favorable to study in transmission due to its large scale height(370 km). We derive the transmission spectrum of WASP-4b using fourtransits observed with the MOS technique. We demonstrate repeatableresults across multiple epochs with GMOS, and derive a combinedtransmission spectrum at a precision about twice above photon noise,which is roughly equal to one atmospheric scale height. The transmissionspectrum is well fitted with a uniform opacity as a function ofwavelength. The uniform opacity and absence of a Rayleigh slope frommolecular hydrogen suggest that the atmosphere is dominated by cloudswith condensate grain sizes of ˜1 μm. This result is consistentwith previous observations of hot Jupiters since clouds have been seenin planets with similar equilibrium temperatures to WASP-4b. We describea custom pipeline that we have written to reduce GMOS time-series dataof exoplanet transits, and present a thorough analysis of the dominantnoise sources in GMOS, which primarily consist of wavelength- andtime-dependent displacements of the spectra on the detector, mainly dueto a lack of atmospheric dispersion correction.