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 circumstellar 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 outer 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 adsorption of molecules onto the dust particles increases. At the outermost surface regions, on the other hand, molecules are dissociated by UV radiation. Hence, in disks with a typical age (105–106 yr), molecular abundances have their peak values at some intermediate region. The height at which the abundances reach their peak values varies with species. Radicals, such as CN and C2H, have their peaks at larger heights Z than more stable species such as HCN and NH3. At significant distances from the midplane, the chemistry is also affected by a higher ionization rate due to X-rays, if the central star is a strong X-ray emitter, while, in the midplane, cosmic-rays are the main sources of ionization. The abundances of some species (e.g. HCN) are enhanced by a higher ionization rate. Integrating molecular abundances perpendicular to the midplane, we obtain column densities for molecules, and their radial distribution. Column densities 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 field 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.