Measuring the structure of Fomalhaut's dusty debris belt via a fortuitous stellar occultation

Fomalhaut, a young nearby A4V star, is surrounded by an inclined dust debris belt, directly imaged in optical scattered light with HST. The proper motion and parallax of Fomalhaut will move this belt across an 18.5 magnitude background star in the next decade. This presents a unique and fortuitous opportunity to probe the dust structure and confirm disk dynamical theory to unprecedented resolution. In Cycle-21 we obtained HST/STIS spectra of the background star during the diffuse phase of the debris disk occultation. We propose obtaining HST/STIS spectra during the thick (Cycle-24 to 26), and pristine (~2021 with JWST) phases of occultation as the debris disk moves across the target background star. This is a rare opportunity to measure - for the first time - the dust optical depth structure in a known planet-forming system, providing a valuable constraint on current planet formation theories. This experiment will act as a pathfinder for this type of science for future observations with JWST and future ELTs.