A SEA BASS on the Exoplanet HD 209458b

We present here the first application of Stellar and Exoplanetary Atmospheres Bayesian Analysis Simultaneous Spectroscopy (SEA BASS) on real data sets. SEA BASS is a scheme that enables the simultaneous derivation of four-coefficient stellar limb-darkening profiles, transit depths, and orbital parameters from exoplanetary transits at multiple wavelengths. It relies on the wavelength independence of the system geometry and on the reduced limbdarkening effect in the infrared. This approach has been introduced by Morello et al. (without the SEA BASS acronym), who discuss several tests on synthetic data sets. Here, we (1) improve on the original algorithm using multiple Spitzer/InfraRed Array Camera passbands and a more effective set of geometric parameters, (2) demonstrate its ability with Hubble Space Telescope /Space Telescope Imaging Spectrograph data sets by (3) measuring the HD 209458 stellar limb-darkening profile over multiple passbands in the 290-570 nm range with sufficient precision to rule out some theoretical models that have been adopted previously in the literature, and (4) simultaneously extracting the transmission spectrum of the exoplanet atmosphere. The higher photometric precision of the next-generation instruments, such as those on board the James Webb Space Telescope, will enable modeling the star-planet systems with unprecedented detail, and increase the importance of SEA BASS for avoiding the potential biases introduced by inaccurate stellar limb-darkening models.