Early Phases of Protoplanetary Disk Evolution

It is widely accepted that planetary systems form from protoplanetary disks, and observations of the dust reveal significant grain growth over timescales of a few million years. However, we know little about the gas processing in the first 10-20 Myr of disk evolution. This is the phase where protoplanetesimals form and accrete into planetary cores. One outstanding question is whether gas dispersal is coeval with the formation of planetesimals. If the gas dispersal would precede the dust evolution, the formation of gas giant planets through the standard core-accretion scenario would be impeded. HST's contribution to our understanding of A disk dispersal processes will be summarized and the [O I] 6,300 angstrom line as tracer of disk structure in early phases of disk evolution discussed.