1998A&A...331..317T

We present the results obtained with a radiative transfer model to investigate the excitation of OH molecules. Fluxes of far-infrared rotational lines of OH are calculated in terms of a radiatively excited envelope and of a radiative transfer model including collisional and radiative excitation. The agreement with the OH far-infrared lines observed with ISO is satisfactory. The 34.6µm line is found to be in absorption as observed. We also predict that the 53.3µm line should be seen in absorption and also contributes to the maser pumping process. Maser pumping efficiencies are estimated from the model and compared to the values determined from recent OH far-infrared observations with ISO and OH maser data.