The excitation of water in the S140 photon dominated region

Abstract

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 ∼350 K (i.e., 8 levels), as well as line intensities for various transitions. Homogeneous and inhomogeneous models are presented with densities of 104-10 5 cm-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 ∼10-13 erg cm-2 s-1 sr-1 to a few times 10-6 erg cm -2 s-1 sr-1. 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). © ESO 2005.