Example of the fit of the OH 119 μm doublet with absorption (upper), emission (middle) and P-Cygni (bottom) profile. The velocity is relative to the blue component of the doublet (at 119.233 μm) at the systemic velocity inferred from the CO(2–1) line. The best-fit model is shown in magenta, while the orange models are the results of the 50 Monte Carlo realisations that illustrate the uncertainty of the fit. Solid lines show the model convolved with the instrumental resolution, while dashed lines indicate the intrinsic (deconvolved) model. Blue and red mark the two components of the fit. The vertical dotted, dashed and dot-dashed lines show the v₅₀, v₈₄ and v₉₈ percentile velocities, respectively, derived from the absorption (blue) and/or emission (red) profiles. Grey dashed lines at ∼2000 km s⁻¹ show the position of the ¹⁸OH 120 μm doublet and the CH⁺(3-2) absorption line.