Physical models of photon - dominated regions : influence of clumpiness and geometry for S 140

Model calculations for the photon-dominated region S 140 are presented. The overall geometry and an inhomogeneous density distribution are incorporated and the chemical and thermal balance is solved in two and three dimensions. The observed widespread neutral carbon emission in the extended molecular cloud is investigated for a range in clump size and volume filling factor. It is found that the clump volume filling factor needs to be less than 30% and the clump size larger than 0.2 pc in order to reproduce the observed [CI] data and their correlationwith 13CO. Because of the presence of a dense core near the edge of the cloud, a large part of the stellar radiation cannot penetrate the extended molecular cloud. Therefore, to explain the [CI] extent, the star must be located closer to the sun, i.e. off the direction defined by the line center cloud – dense core. The dense core itself contains three embedded young stars, which influence the source morphology of the dense interface region. Detailedmodels including these internal sources are constructed and the results compare well with high spatial resolution observations. Maps of the [C II], [C I], [O I] and CO line emission are presented for various cases, and their relation with the underlying geometry and density distribution is discussed. It is found that geometrical effects play a central role in the resulting intensity and morphology of line maps.