2006A&A...453..615P

H₂O is a key ingredient in many interstellar environments, like photon dominated regions and star forming clouds. It plays an important role in the oxygen chemistry and can act as a coolant in dense interstellar clouds and shocks. Observations and modelling of water lines thus provide powerful diagnostics of the physical conditions in interstellar emission zones. A radiative transfer method for the treatment of molecular lines is presented. We apply this method to previous SWAS and ISO observations of water vapor in the source S140 in order to make models to plan for, and to interpret, HIFI data. Level populations are calculated with the use of a three-dimensional (multi-zone) escape probability method and with a long characteristic code that uses Monte-Carlo techniques with fixed directions. Homogeneous and inhomogeneous models are used to compute the differences between H₂O line profiles across the S140 region. We find that when an outflow or infall velocity field with a gradient of a few km/s is adopted, line profiles with a FWHM of 6km/s are found, in agreement with observations. Inhomogeneous models are favoured to produce a single-peaked line profile. When zooming in on smaller regions within the PDR, the shapes of the line profiles start to differ due to the different temperature and density distributions there. The embedded sources are traced by high excitation lines, e.g., 3₂₁-2₂₁, 3₀₃-2₁₂, 2₁₂-1₀₁ and 2₂₀-1₁₁. The computed intensities are roughly consistent with existing ISO observations. Water emission in a PDR source like S140 requires a combination of a pure PDR and an embedded source in order to match the observations. Because of its good angular resolution, HIFI will be able to distinguish between a dense star forming region or a more diffuse gas component. It is therefore important for future observing programs to consider both in their predictions of the emission characteristics of water in these environments.