Demonstration of the multi-component fitting procedure, taking the Mon R2 Pos 1 spectrum as an example. Each plot (a–c) is structured in the same way. Left top: fitted model in green and the observed spectrum in red. Middle left: zoom-in vertically of the fitted model and the observed spectra to better show the [¹³C II] satellites. Left bottom: residual between the observed spectra and the model. Right top: fitted background emission component in blue, the resulting background emission model from the addition of all the components in cyan and the observed spectrum in red. Right bottom: optical depth of each absorbing foreground component (inverted scale) in pink. a: fitting of the [¹³C II] emission, masking the velocity range of the [¹²C II] emission. b: fitting of the remaining [¹²C II] background emission. c: fitting of the foreground absorbing components.