Observing Disk Accretion in Action

Physical processes that redistribute or remove angular momentum from protoplanetary disks can drive mass accretion onto the star and affect the outcome of planet formation. Despite ubiquitous evidence that disks accrete, the process(es) responsible remain unclear. Here we present new results that appear to show disk accretion in action via rapid inflow of molecular gas at the surface of a protoplanetary disk. High-resolution mid-infrared spectroscopy of the Class I source GV Tau N reveals a rich redshifted absorption spectrum of individual lines of acetylene, hydrogen cyanide, ammonia, and water. The properties of the absorption indicate that the flow carries a significant accretion rate, comparable to stellar accretion rates of active T Tauri stars. The results may provide evidence for supersonic "surface accretion flows," which have been found in MHD simulations of magnetized disks.