Astron. Astrophys. Suppl. Ser. 137, 323-336
T. Thissen - H. Spiecker - P. Andresen
Send offprint request: T. Thissen,
e-mail: tthissen@physik.uni-bielefeld.de
Correspondence to: Fakultät Physik, Universität Bielefeld, Universitätsstr. 25, D-33615 Bielefeld, Germany.
Angewandte Laserphysik, Universität Bielefeld, Universitätsstr. 25, D-33615 Bielefeld, Germany
Received August 13, 1998; accepted March 22, 1999
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 -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.
Key words: ISM: OH -- masers -- hyperfine population -- W3(OH) -- photodissociation -- HII regions
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