Probing protoplanetary disk evolution with the HI 21 cm line

Little is known about the gas disk dispersal timescales in the planet formation process. Disks have a complex chemical structure and a wide range of excitation conditions, making the interpretation of line observations difficult. Here, we use detailed chemo-physical models to predict the Hi abundance for a wide range of disk evolutionary stages. Hi traces the surface layers and the Hi 21 cm line - if detected - can be used to study the effects of UV, x-ray irradiation and evaporation in addition to the disk kinematics. We use a simple radiative transfer approach to convert the model disk parameters into predicted 21 cm line maps and spectral profiles. These are compared to upper limits on the 21 cm emission from the Australia Telescope Compact Array and Giant Meterwave Radio Telescope to obtain limits on the total disk mass for Herbig Ae, T Tauri and debris disk stars. We also discuss more recent VLA observations of the protoplanetary disks in Orion, where we looked for the Hi 21 cm line in absorption against the bright nebula background. As an outlook, we briefly address the capabilities of the planned Square Kilometer Array for imaging the surfaces of disks during the planet formation stage.