Role of H2O in molecular clouds: line transfer and thermal balance in warm dusty medium

The heating of gas in optically thick molecular clouds with embedded luminosity sources is studied. The dust is heated by the flow of infrared luminosity out of the cloud and assumes temperatures which range from 10 to 3000K. The gas is weakly coupled to the radiation field and is generally cooler than the dust. The main heating mechanisms for the gas are by direct collisions with the dust grains and by collisions with H2O molecules, which are radiatively excited by dust infrared emission. The latter process is quantitatively treated in a simple model of the thermal balance in such clouds. The effects of each cloud parameter on the thermal structure and the emergent molecular line intensities are studied for a linearly expanding cloud. H2O lines appear both in emission at long wavelengths (lambda approx.> 100 μM) and in absorption at shorter wavelengths. Line shapes often show the asymmetric P-Cygni features, and line strengths range up to approx. 10-3erg cm-2s-1sr-1; H2O lines are sensitive to cloud optical depth and dust temperature. CO and Cl lines appear in emission, and the strongest lines (approx. 10-4erg cm-2s-1sr-1) are at longer wavelengths than the H2O lines