The Formation and Depletion of Molecules in Dense Interstellar Clouds

The rates of a number of homogeneous gas phase reactions likely to occur in dense interstellar clouds composed primarily of molecular hydrogen are considered. Over 100 reactions are included; of these, more than half have known rate constants. These reactions are sufficient to produce and destroy polyatomic species such as H2O, HCN, NH3, and H2CO. Steady-state abundances of about 35 species are calculated for clouds of density 1 0 < < 106 cm -3. Calculations are performed for cosmic-ray-induced ionization rates H3 = 10-17 and 10-18 1 Typical times for reaching steady-state (flH2 = 10 cm - 3, H3 = 10-17 s-1) vary for different species; e.g., H2 + 0.05 day, H3 + 0.3 year, 1130 3 x 10 years. Comparisons of calculated and observed column densities are presented for a number of regions. Subject headings: excitation and ionization - molecules, interstellar - nebulae