The Formation of Molecules in Contracting Interstellar Clouds

The abundances of atoms, molecules and ions in contracting interstellar clouds are investigated in the wide ranges of density (from 10 em-' to 107 em-') and optical depth. Abundances of molecules are not in a steady state in optically thick stages because their reaction time scales are very long (10 12·'-10"·' sec) compared with the contraction time scales. At some stage of contraction the abundances of neutral molecules become frozen, and the frozen abundances are considerably different from the steady.state abundances. The frozen abundances are mainly determined by the contraction time scale of the cloud. Especially, molecules containing carbon except for CO are less abundant for the cloud contracting more slowly.