Evidence for the effect of the interstellar far UV-radiation on circumstellar shells

The interstellar UV field at 1565 Å is calculated around nearby OH/IR sources. The front-back asymmetry observed in the 1612 MHz maser line profile is well correlated with anisotropy of the interstellar UV flux. For some sources the spatial positions of the 1612 MHz masers are confined to the position angles for which stronger UV radiation occurs. These facts strongly support the theory of the photoproduction of OH from H₂O induced by ambient interstellar UV photons penetrating the circumstellar shell. A simple model of the 1612 MHz maser with OH photoproduction suggests that the influence of the UV field on the observed maser profiles is governed by the mass loss rate and the relative abundances of OH and H₂O molecules.