EVOLUTION OF MOLECULAR ABUNDANCE IN PROTOPLANETARY DISKS

We investigate the evolution of molecular abundance in quiescent proto planetary disks that are presumed to be around weak-lined T Tauri star s. In the region of surface density less than 10(2) g cm(-2) (distance from the star greater than or similar to 10 AU in the minimum-mass so lar nebula), cosmic rays are barely attenuated even in the midplane of the disk and produce chemically active ions such as He+ and H-3(+). T hrough reactions with these ions, CO and N-2 are finally transformed i nto CO2, NH3, and HCN. In the region where the temperature is low enou gh for these products to freeze onto grains, a considerable amount of carbon and nitrogen is locked up in the ice mantle and is depleted fro m the gas phase in a timescaIe of less than or similar to 3 x 10(6) yr . Oxidized (CO2) ice and reduced (NH3 and hydrocarbon) ice naturally c oexist in this part of the disk. The molecular abundance both in the g as phase and in the ice mantle varies significantly with the distance from the central star.