2009A&A...494..637P

We investigate the effect of the physical environment on water and ammonia abundances across the S140 photodissociation region (PDR) with an embedded outflow. We used the Odin satellite to obtain strip maps of the ground-state rotational transitions of ortho-water and ortho-ammonia, as well as CO(5-4) and ¹³CO(5-4) across the PDR, and H₂¹⁸O in the central position. A physi-chemical inhomogeneous PDR model was used to compute the temperature and abundance distributions for water, ammonia, and CO. A multi-zone escape probability method then calculated the level populations and intensity distributions. These results are compared to a homogeneous model computed with an enhanced version of the RADEX code. H₂O, NH₃, and ¹³CO show emission from an extended PDR with a narrow line width of ∼3km/s. Like CO, the water line profile is dominated by outflow emission, but mainly in the red wing. Even though CO shows strong self-absorption, no signs of self-absorption are seen in the water line. The H₂¹⁸O molecule is not detected. The PDR model suggests that the water emission arises mainly from the surfaces of optically thick, high-density clumps with n(H₂)>10⁶cm^–3 and a clump water abundance, with respect to H₂, of 5x10^–8. The mean water abundance in the PDR is 5x10^–9 and between ∼4x10^–8-4x10^–7 in the outflow derived from a simple two-level approximation. The RADEX model points to a somewhat higher average PDR water abundance of 1x10^–8. At low temperatures deep in the cloud, the water emission is weaker, likely due to adsorption onto dust grains, while ammonia is still abundant. Ammonia is also observed in the extended clumpy PDR, likely from the same high density and warm clumps as water. The average ammonia abundance is about the same as for water: 4x10^–9 and 8x10^–9 given by the PDR model and RADEX, respectively. The differences between the models most likely arise from uncertainties in density, beam-filling, and volume-filling of clumps. The similarity of water and ammonia PDR emission is also seen in the almost identical line profiles observed close to the bright rim. Around the central position, ammonia also shows some outflow emission, although weaker than water in the red wing. Predictions of the H₂O 1_1,0-1_0,1 and 1_1,1-0_0,0 antenna temperatures across the PDR are estimated with our PDR model for the forthcoming observations with the Herschel Space Observatory.