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The short period ($0.94$-day) transiting exoplanet WASP-19b is an exceptionaltarget for transmission spectroscopy studies, due to its relatively largeatmospheric scale-height ($\sim 500$ km) and equilibrium temperature ($\sim2100$ K). Here we report on six precise spectroscopic Magellan/IMACSobservations, five of which target the full optical window from $0.45-0.9\mu$mand one targeting the $0.4-0.55\mu$m blue-optical range. Five of these datasetsare consistent with a transmission spectrum without any significant spectralfeatures, while one shows a significant slope as a function of wavelength,which we interpret as arising from photospheric heterogeneities in the star.Coupled with HST/WFC3 infrared observations, our optical/near-infraredmeasurements point to the presence of high altitude clouds in WASP-19b'satmosphere in agreement with previous studies. Using a semi-analyticalretrieval approach, considering both planetary and stellar spectral features,we find a water abundance consistent with solar for WASP-19b and strongevidence for sub-solar abundances for optical absorbers such as TiO and Na; nostrong optical slope is detected, which suggests that if hazes are present,they are much weaker than previously suggested. In addition, two spot-crossingevents are observed in our datasets and analyzed, including one of the firstunambiguously detected bright spot-crossing events on an exoplanet host star.

ACCESS: A featureless optical transmission spectrum for WASP-19b from Magellan/IMACS