ACCESS: a featureless optical transmission spectrum for WASP-19b from Magellan/IMACS
Fortney, Jonathan J.; López-Morales, Mercedes; Jordán, Andrés; Espinoza, Néstor; Wilson, Paul A.; Apai, Dániel; Bixel, Alex; Rackham, Benjamin V.; Lewis, Nikole K.; Grimm, Simon L.; Fraine, Jonathan; Rodler, Florian; Osip, David J.; Hoeijmakers, Jens; McGruder, Chima; Weaver, Ian
Germany, Chile, United States, Switzerland, Netherlands
Abstract
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 (∼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 to 0.9 μm and one targeting the 0.4-0.55 μm 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.