MOLECULAR EVOLUTION IN PLANET-FORMING CIRCUMSTELLAR DISKS

We have investigated the evolution of molecular abundance in circumste llar disks around young low-mass stars, which are considered to be the formation sites of planetary systems. Adopting the standard accretion disk model, we investigated molecular evolution mainly in the accreti on phase. In the region of surface density less than 10(2) g cm(-2) (d istance from the star greater than or similar to 10 AU), cosmic rays a re barely attenuated, even in the midplane of the disk, and produce ch emically active ions such as He+ and H-3(+). We found that a considera ble amount of CO and N-2, the initial dominant components of the disk, is transformed into CO2, CH4, NH3 and HCN through reactions with thes e ions. Where the temperature is low enough for these products to free ze onto grains, they are selectively 'locked up' and accumulate in the ice mantle. As the matter accretes towards inner warmer regions, the ice mantle evaporates. The desorbed molecules, such as CH4, are transf ormed into larger and less volatile molecules by reactions in the gas phase. The molecular abundance, both in the gas phase and in the ice m antle, depends crucially on the temperature and thus varies significan tly with the distance from the central star. If the ionization rate an d the grain size in the disk are the same as those in molecular clouds , the timescale of the molecular evolution, in which CO and N-2 are tr ansformed into other molecules is, ca. 10(6) years, slightly less than the life time of the disk. The timescale of molecular evolution is le ss for higher ionization rates and greater for lower ionization rates or larger grain size. We have compared our results with the molecular composition in comets, the most primitive bodies in our solar system. The molecular abundance derived from our model reproduces the coexiste nce of oxidized ice and reduced ice, as observed in comets. Our model also suggests that comets formed in different regions of the disk will have different molecular compositions.