Molecular evolution in protoplanetary disks - Two-dimensional distributions and column densities of gaseous molecules

We investigate the two-dimensional (R, Z) distribution of molecules in circ umstellar disks around young stellar objects. In the Z-direction, the disk can be schematically divided into three layers: the midplane, the outermost surface region, and the intermediate region. On the midplane, in cool oute r regions of a disk, most of the molecules are adsorbed onto grains within a short time and depleted from the gas phase owing to the high density. As the height Z increases, the density decreases and the time scale for adsorp tion of molecules onto the dust particles increases. At the outermost surfa ce regions, on the other hand, molecules are dissociated by UV radiation. H ence, in disks with a typical age (10(5)-10(6) yr), molecular abundances ha ve their peak values at some intermediate region. The height at which the a bundances reach their peak values varies with species. Radicals, such as CN and C2H, have their peaks at larger heights Z than more stable species suc h as :HCN and NH3. At significant distances from the midplane, the chemistr y is also affected by a higher ionization rate due to X-rays, if the centra l star is a strong X-ray emitter, while, in the midplane, cosmic-rays are t he main sources of ionization. The abundances of some species (e.g. HCN) ar e enhanced by a higher ionization rate. Integrating molecular abundances perpendicular to the midplane, we obtain c olumn densities for molecules, and their radial distribution. Column densit ies of selected molecules such as HNC and NH3 are particularly sensitive to the total column density of the disk and the variation in UV radiation fie ld due to the growth and sedimentation of dust particles. Our results show reasonable agreement with molecular abundances in the DM Tau disk estimated from radio observations.