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Context: observations of water lines are a sensitive probe of the geometry,dynamics and chemical structure of dense molecular gas. The launch of Herschelwith on board HIFI and PACS allow to probe the behaviour of multiple waterlines with unprecedented sensitivity and resolution. Aims: we investigate thediagnostic value of specific water transitions in high-mass star-formingregions. As a test case, we apply our models to the AFGL2591 region. Results:in general, for models with a constant water abundance, the ground state lines,i.e., 1_(10)-1_(01), 1_(11)-0_(00), and 2_(12)-1_(01), are predicted inabsorption, all the others in emission. This behaviour changes for models witha water abundance jump profile in that the line profiles for jumps by a factorof ~10-100 are similar to the line shapes in the constant abundance models,whereas larger jumps lead to emission profiles. Asymmetric line profiles arefound for models with a cavity outflow and depend on the inclination angle.Models with an outflow cavity are favoured to reproduce the SWAS observationsof the 1_(10)-1_(01) ground-state transition. PACS spectra will tell us aboutthe geometry of these regions, both through the continuum and through thelines. Conclusions: it is found that the low-lying transitions of water aresensitive to outflow features, and represent the excitation conditions in theouter regions. High-lying transitions are more sensitive to the adopted densityand temperature distribution which probe the inner excitation conditions. TheHerschel mission will thus be very helpful to constrain the physical andchemical structure of high-mass star-forming regions such as AFGL2591.

Modeling the water line emission from the high-mass star-forming region AFGL2591