Mapping Spots on the Stellar Surface with Exoplanet Transmission Spectrum Observations

AU Microscopii is among the most heavily spotted stars while hosting one of the best targets for atmospheric characterization of sub-Neptune worlds, given the temperature (3700 K), proximity (9.7 pc), and age (20-25 Myr) of the system. HST/WFC3 transmission spectrum observations of its innermost planet, AU Mic b, are expected to exhibit strong atmospheric absorption, but these signals are obscured by various forms of stellar activity. In this talk, I will discuss a method of decontaminating the transmission spectrum that uses chromatic information in the spectroscopic light curves to characterize the stellar surface alongside the planetary atmosphere.