1996A&A...311..690L

In this paper we present a model of time-dependent gas-phase chemistry with complex molecules in a spatially inhomogeneous cloud which is exposed on one side to an isotropic radiation field. In addition to the shielding afforded by the dust particles, we consider the shielding due to H₂ and CO. The cloud is divided into 43 slabs, which are defined so that the photodissociation rate of CO, determined in a simplified chemical model, is halved as one proceeds to the next slab. The hydrogen number density is a function of cloud depth, ranging from 10²cm^–3 at the cloud surface to ≃10⁴cm^–3 at the cloud center, and varying as (1-cr)^–2 where r is the depth into the cloud. The cloud temperature is kept constant at 30K. The chemical/radiative transfer model is solved for each slab, starting from the outermost slab. Models with three different sets of initial conditions regarding hydrogen are presented here. In the first model, we start with atomic hydrogen in all slabs, while in the second, we start with molecular hydrogen. The third model utilizes an H/H₂ fraction ranging from large in the outer slabs to small in the inner ones. The variations in fractional abundance and integrated column density for selected atoms (H, C) and molecules as functions of cloud depth at different times are discussed. A comparison between total integrated column densities with observations in TMC1 is given. A plausible dynamic scenario for the material in and in front of TMC1 based on our chemical results is suggested. Alternative total integrated column densities are obtained via a weighting procedure designed for unresolved distant sources in which the outer slabs are weighted more heavily than the inner ones due to their larger radii.