Top: optical depth derived from observation towards 2MASS J18170957-0814136 from Boogert et al. (2011) (black) and the synthetic optical-depth spectrum using the column densities derived from the latter (orange). Middle: synthetic optical depth from Nautilus predictions; the ‘cold’ model where the dust temperature (T_dust) was computed with the parametrisation from Hocuk et al. (2017) (T_dust = 6.6 K) is shown in grey; the ‘intermediate’ model (T_dust = 10.3 K) is shown in cyan; the ’warm’ model where T_dust was obtained from Herschel data (T_dust = 13.2 K) is shown in red. Bottom: absolute residuals from subtraction between observations and synthetic spectra presented above.