2005A&A...440..559P

We consider the excitation of water in the Photon Dominated Region (PDR) S140. With the use of a three-dimensional escape probability method we compute the level populations of ortho- and para-H₂O up to ∼350K (i.e., 8 levels), as well as line intensities for various transitions. Homogeneous and inhomogeneous models are presented with densities of 10⁴-10⁵cm^–3 and the differences between the resulting intensities are displayed. Density, temperature, and abundance distributions inside the cloud are computed with the use of a self-consistent physi-chemical (in)homogeneous model in order to reproduce the line intensities observed with SWAS, and to make predictions for various lines that HIFI will probe in the future. Line intensities vary from ∼10^–13erg/cm²/s/sr to a few times 10^–6erg/cm²/s/sr. We can reproduce the intensity for the 1₁₀->1₀₁ line observed by the SWAS satellite. It is found that the 2₁₂->1₀₁ line is the strongest, whereas the 3₁₂->2₂₁ line is the weakest, in all the models. It is found that the 1₁₀->1₀₁ line probes the total column, while higher excitation lines probe the higher density gas (e.g., clumps).