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

Abstract

The short period ($0.94$-day) transiting exoplanet WASP-19b is an exceptional\ntarget for transmission spectroscopy studies, due to its relatively large\natmospheric scale-height ($\\sim 500$ km) and equilibrium temperature ($\\sim\n2100$ K). Here we report on six precise spectroscopic Magellan/IMACS\nobservations, five of which target the full optical window from $0.45-0.9\\mu$m\nand one targeting the $0.4-0.55\\mu$m blue-optical range. Five of these datasets\nare consistent with a transmission spectrum without any significant spectral\nfeatures, while one shows a significant slope as a function of wavelength,\nwhich we interpret as arising from photospheric heterogeneities in the star.\nCoupled with HST/WFC3 infrared observations, our optical/near-infrared\nmeasurements point to the presence of high altitude clouds in WASP-19b's\natmosphere in agreement with previous studies. Using a semi-analytical\nretrieval approach, considering both planetary and stellar spectral features,\nwe find a water abundance consistent with solar for WASP-19b and strong\nevidence for sub-solar abundances for optical absorbers such as TiO and Na; no\nstrong optical slope is detected, which suggests that if hazes are present,\nthey are much weaker than previously suggested. In addition, two spot-crossing\nevents are observed in our datasets and analyzed, including one of the first\nunambiguously detected bright spot-crossing events on an exoplanet host star.\n