Chemical and dynamical evolution of protostellar clouds: Chemistry at early stages of collapse

A self-consistent model for the dynamical and chemical evolution of a proto stellar cloud is used to study the chemical changes caused by the passage o f a compression wave into the cloud. This compression wave is induced by th e thermal pressure of the cloud outer envelope heated by UV photons. Gas-ph ase and grain-surface chemical reactions to produce simple molecules of the carbon, nitrogen, and oxygen families are shown to be most active precisel y in the wave region. The formation of molecules is most effective in the i nner part of the compression wave, while its outer part is dominated by the evaporation of icy grain mantles. The chemical coupling between gas and du st causes a local increase in the number densities of chemically active rad icals, such as C, N, O, NH, and OH, and hydrogen-saturated CH4, NH3, and H2 O molecules. It has been found that the chemical evolution in the compressi on-wave region can account for the observed abundance of water and ammonia in interstellar clouds.