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

Abstract

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, 0, S and Si) and consisted of the surface recombination of hydrogen on grains and2884gasphase reactions of 234 species. For the model adopted (nH=10'cm-, T=30K, CH= w-" s-1 per H atom), observed molecules are produced successfully and attain their steady-state abundances at t= "-'2 X 10 16 s, which is longer than the evolutional time scale of dense clouds. Molecules containing carbon (except carbon in C-0 bond) have peak abundances at t= '""10 10 s, when most C atoms are consumed in forming CO molecules. The C/CO ratio is a good indicator of molecular evolution in dense clouds. 1 . 7 x w- + CH,CHCCH-7CH,CHCHCH, + + H, 1. 7xw- CH,CHCCH+ + H,-->C,H, + + H 1.5x10- C,H. + + H,--.c,H, + + H 1. 5x w- CaN++ H,-->HCCCN+ + H 1.