Schematic representation of a given source component. It consists of (i) a central source of far-IR emission characterized by its radius (Rint), dust temperature (Tdust), and optical depth at 100 μm along a radial path (τ100), and (ii) the surrounding (outflowing) gas, with external radius Rout, which is mixed with dust. The gas expands radially with a velocity field v(r) and H nuclei density n(r), such that nOH × r2 × v is constant (i.e. the mass-outflow rate is constant). The dust in the outflowing envelope has τ50 = 0.5 along a radial path, and Tdust ~ r-0.4. In some models, departures from spherical symmetry are simulated by calculating the emerging fluxes only for impact parameters lower than pf (i.e. between the two dashed lines). The blue and red curves (and grayscale) show the isocontours of line-of-sight velocities (blue: approaching; red: receding) for a decelerating outflow; the darkest colors correspond to the highest (approaching or receding) velocities. In our best-fit models for the excited OH lines (the HVC component in Sect. 3.1.2), the outflowing envelope is less extended than in this representation (Rout/Rint ≲ 1.5) and is collimated (pf < Rout).
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