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

The short-period (0.94-d) transiting exoplanet WASP-19b is an exceptional target for transmission spectroscopy studies, due to its relatively large atmospheric scale height (similar to 500 km) and equilibrium temperature (similar to 2100 K). Here, we report on six precise spectroscopic Magellan/IMACS observations, five of which target the full optical window from 0.45 to 0.9 mu m and one targeting the 0.4-0.55 mu m blue-optical range. Five of these data sets are consistent with a transmission spectrum without any significant spectral features, 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-infrared measurements point to the presence of high altitude clouds in WASP-19b's atmosphere in agreement with previous studies. Using a semi-analytical retrieval approach, considering both planetary and stellar spectral features, we find a water abundance consistent with solar for WASP-19b and strong evidence for sub-solar abundances for optical absorbers such as TiO and Na; no strong optical slope is detected, which suggests that if hazes are present, they are much weaker than previously suggested. In addition, two spot-crossing events are observed in our data sets and analysed, including one of the first unambiguously detected bright-spot-crossing events on an exoplanet host star.