Time-dependent chemistry - I. Modelling of a static cloud

Abstract

A computer program has been developed in order to study the chemical evolution in interstellar clouds. This time-dependent treatment allows both the gas kinetic temperature and the total particle density to be rapidly varying functions of time. Such modelling would be appropriate for hot post-shock gas, circumstellar material around cool stars and expanding or collapsing clouds. In this paper we describe the modelling procedure and explain some general results obtained using a basic time-dependent programme for a quiescent, static cloud of constant density and kinetic temperature. In each cloud model considered the system of molecular abundances reaches equilibrium within a few million years of evolution and there is no evidence of any quasi-equilibrium stages at earlier times. The results of each model are extremely sensitive to the input values used for elemental abundances and the initial fraction of H2, but the equilibrium values of the molecular densities do not depend on the initial distribution of carbon amongst the species C, C+ or CO. Ion molecule chemistry is found to be efficient only for values of |$n(\text H_2)/n(\text H + 2\text H_2)$| ≳ 20 per cent. The value of the cosmic ionization rate for H atoms is also closely related to the overall rate of molecule formation. Among the results are the following: (i) Diffuse clouds, in which the chemistry is driven partially by photoionization reach their equilibrium after a few thousand years, with the exception of a few molecules (CO, N2) which are formed by slower reactions and hence require longer times to reach equilibrium. (ii) In denser clouds the approach to equilibrium is a complex function of the chemical network and the initial description. Unless the entire chemical network and the initial conditions are well defined, it is not possible-to determine the age of observed objects from their chemical composition. The abundances between t = 0 yr and t = 106 yr are functions of the initial parameters, such as elemental abundances, which are not well determined. (iii) Different species of molecule reach peak abundances at different cloud densities. Therefore, for a cloud with a non-constant density–radius law, any line-of-sight has a variable fractional abundance of each species. (iv) In general, the results obtained with this programme agree with those of other models and, broadly, with the observations. The lack of close agreement between theory and observation for certain species most likely indicates an incomplete chemistry or incorrect choices for the rate coefficients in the reaction scheme. For this scheme only about 25 per cent of the reactions have measured values for the rate coefficients.