MONTE-CARLO MODELS OF THE PHYSICAL AND CHEMICAL-PROPERTIES OF INHOMOGENEOUS INTERSTELLAR CLOUDS

Two-dimensional model calculations for inhomogeneous interstellar clou ds are presented. These models can accommodate an arbitrary geometry a nd density structure. The equation for radiative transfer in the conti nuum is solved by means of a Monte Carlo method. Self-shielding of H-2 and CO, as well as the shielding of CO by H-2, is treated in the one- line approximation. The detailed temperature structure of the model cl oud is calculated taking all the relevant heating and cooling processe s into account. Models are presented for diffuse and translucent cloud s with A(V) = 1 - 5 mag. The inhomogeneous nature of interstellar clou ds influences the abundances and excitation of atomic and molecular sp ecies. Specifically, it is shown that C+ is found mostly in the interc lump medium, whereas (CO)-C-13 traces mainly the dense clumps. Atomic carbon, C, has its largest abundance in the surface layers of the clum ps. The calculated column densities for inhomogeneous and homogeneous models are compared, and it is shown that the inhomogeneous models lea d to a large increase (up to a factor of 5) in the CO column density b ut to a decrease in the abundances of species that do not shield thems elves against photodissociation. The inhomogeneous structure also affe cts the temperature structure, resulting in enhanced temperatures deep er into the cloud. The contribution of small grains and large molecule s like Polycyclic Aromatic Hydrocarbons (PAHs) is significant at the e dge of the cloud. The observational consequences for the [CII] 158 mu m and [CI] 609 mu m line maps are discussed.