Dust opacity models (extinction/absorption coefficient per g of dust) used in this paper. Black lines show the absorption (solid) and extinction (dashed) coefficients of the modified OH5a model (Ossenkopf & Henning 1994). The gray line shows the OH1 model (see Sect. 5.1.3; for clarity, only the absorption coefficient spectrum is shown). Vertical dashed lines mark the wavelength range of the original OH models, i.e., values outside this range are extrapolated (see text). Arrows indicate the wavelength bands of the dust emission data used in this paper.