ACCESS: A featureless optical transmission spectrum for WASP-19b from Magellan/IMACS
The short period (0.94-day) transiting exoplanet WASP-19b is an exceptional target for transmission spectroscopy studies, due to its relatively large scale-height (500 km) and equilibrium temperature (2100 K). Here we report on six precise spectroscopic Magellan/IMACS observations, five of which target the full optical window from 0.45-0.9um and one targeting the 0.4-0.55um blue-optical range. Five of these datasets are consistent with a featureless transmission spectrum, 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. 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 of optical absorbers such as TiO/VO, Na and K; 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, including one of the first unambiguously detected bright spot-crossing events on an exoplanet host star.