Synthesis of complex molecules in dense interstellar clouds via gas-phase chemistry - A pseudo time-dependent calculation

A pseudo-time-dependent numerical model of the gas-phase chemistry of dense interstellar clouds has been developed, in order to determine whether gas-phase processes can account for the high degree of chemical activity found in the course of recent observations. The model includes 200 species and over 1800 gas-phase reactions. To simulate chemical processes in the clouds, an initial set of chemical abundances is allowed to evolve under constant physical conditions and in the absence of photodissociations. The results show that under constant conditions complex molecular species are produced in significant amounts, but not under the steady-state conditions which exist when C I abundance is a small fraction of the CO abundance. It is found that the abundances calculated by the model often exceeded observed values. It is suggested that this discrepancy is due to the difference in the time of maximum complex molecular abundances and the time when steady-state is reached (not less than 10 to the 7th years). Some of the observational implications of the data are discussed, and a table listing the calculated abundances is provided.