Modeling the water line emission from the high-mass star-forming region AFGL 2591

Abstract

Context. Observations of water lines are a sensitive probe of the geometry, dynamics and chemical structure of dense molecular gas. The launch of Herschel with on board HIFI and PACS allows to probe the behaviour of multiple water lines with unprecedented sensitivity and resolution. Aims. We investigate the diagnostic value of specific water transitions in high-mass star-forming regions. As a test case, we apply our models to the AFGL 2591 region. Methods. A multi-zone escape probability method is used in two dimensions to calculate the radiative transfer. Similarities and differences of constant and jump abundance models are displayed, as well as when an outflow is incorporated. Results. In general, for models with a constant water abundance, the ground state lines, i.e., 110–101 ,1 11–000 ,a nd 2 12–101 ,a re predicted in absorption, all the others in emission. This behaviour changes for models with a water abundance jump profile in that the line profiles for jumps by a factor of ∼10–100 are similar to the line shapes in the constant abundance models, whereas larger jumps lead to emission profiles. Asymmetric line profiles are found for models with a cavity outflow and depend on the inclination angle. Models with an outflow cavity are favoured to reproduce the SWAS observations of the 110–101 ground-state transition. PACS spectra will tell us about the geometry of these regions, both through the continuum and through the lines. Conclusions. It is found that the low-lying transitions of water are sensitive to outflow features, and represent the excitation conditions in the outer regions. High-lying transitions are more sensitive to the adopted density and temperature distribution which probe the inner excitation conditions. The Herschel mission will thus be very helpful to constrain the physical and chemical structure of high-mass star-forming regions such as AFGL 2591.