COLLISIONAL PUMPING OF H2O MASERS IN STAR-FORMING REGIONS

We have studied the pumping of H2O masers in star-forming regions by collisions with H2 molecules. We have taken the temperature to be 400 K, as is thought appropriate for a region behind high-velocity dissociative shocks, and modeled the maser regions as cylinders. For the radiative transfer we used the Sobolev approximation in all directions. Parameters such as the density, the fractional abundance of H2O, the velocity gradients, and the temperature of the maser region were allowed to vary, and the expected brightness temperature and isotropic photon-emission rate were computed. We have found that the stronger water masers in star-forming regions may not be explained by collisional pumping, unless one is willing to accept very large aspect ratios (length/diameter) for the cylinder or small velocity gradients along the axis of the cylinder. In addition to the 22 GHz water maser, we present results for other maser transitions of water in the submillimeter range. The detection of these masers with submillimeter telescopes could significantly enhance our understanding of the physical conditions in star-forming regions.