Evolution of Molecular Abundance in Protoplanetary Disks

We investigate the evolution of molecular abundance in quiescent protoplanetary disks which are presumed to be around weak-line T Tauri stars. In the region of surface density less than $10^2$ g cm$^{-2}$ (distance from the star $\gtrsim 10$ AU in the minimum- mass solar nebula), cosmic rays are barely attenuated even in the midplane of the disk and produce chemically active ions such as He$^+$ and H$_{3}^+$. Through reactions with these ions CO and N$_2$ are finally transformed into CO$_2$, NH$_3$, and HCN. In the region where the temperature is low enough for these products to freeze onto grains, considerable amount of carbon and nitrogen is locked up in the ice mantle and is depleted from the gas phase in a time scale $\lesssim 3\times 10^6$ yr. Oxidized (CO$_2$) ice and reduced (NH$_3$ and hydrocarbon) ice naturally coexist in this part of the disk. The molecular abundance both in the gas phase and in ice mantle varies significantly with the distance from the central star.Comment: 7 pages latex file (using aas2pp4.sty), 3 figures (ps file), to appear in the Astrophysical Journal Letter