Abstract
A mathematical model for the explanation of OH masers in regions of star formation is presented. The model is based on the assumption that grains of different sizes are present at the border of the HII regions where they are exposed to the heat- and VUV-flux of a new born star. The grains evaporate water because of the heat flux from the central star. The subsequent dissociation of water by VUV together with IR relaxation generates a high OH abundance and yields strong inversion in the OH Λ -doublets for all maser transitions observed in star forming regions. The gain of the observed masers is determined as a function of the photodissociation rates for different evaporation rates. In this first approach IR pumping and collisions are neglected although they may change the quantitative predictions. The results show that gain may be high enough to explain all observed masers near new born stars under reasonable assumptions for the astrophysical conditions found at the border of HII regions. Based on the model predictions are made concerning intensities, locations and variability of the different maser transitions.
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