Molecular Evolution in Interstellar Clouds. I: Ion Chemistry in Dense Clouds

Molecular abundances are calculated time-dependently using a chemical scheme which is constructed to represent the evolutional feature of all molecules containing up to 2 heavy atoms (C, N, O, S and Si) and consisted of the surface recombination of hydrogen on grains and 2884 gas phase reactions of 234 species. For the model adopted (nH=105cm-8, T=30K ζH=10-18 s-1 per H atom), observed molecules are produced successfully and attain their steady-state abundances at t = ∼2 ×1016s, which is longer than the evolutional time scale of dense clouds. Molecules containing carbon (except carbon in C-O bond) have peak abundances at t= ∼1016s, when most C atoms are consumed in forming CO molecules. The C/CO ratio is a good indicator of molecular evolution in dense clouds.