Predicted OH column density in the region where T_gas ≥ 150 K for different initial distributions of nitrogen (columns) and sublimating ices (rows). T_gas ≥ 150 K is the temperature range probed by the Herschel PACS observations. The horizontal black dotted lines indicate the upper limit on the OH column density derived from the observations 150 and 200 K. Similarly to H₂O, all models overpredict the upper limit on the OH column density. This could be due to optically thick dust at 51 − 220 μm hiding part