Collisional pumping of H2O masers in star-forming regions

The pumping of H2O masers in star-forming regions by collisions with H2 molecules has been studied. It is found that the strong water masers in star-forming regions may not be explained by collisional pumping, unless very large aspect ratios (length/diameter) for the cylinder or small velocity gradients along the axis of the cylinder are assumed. In addition to the 22 GHz water maser, results are presented for other maser transitions of water in the submillimeter range. The detection of these masers with submillimeter telescopes could significantly enhance the understanding of the physical conditions in star-forming regions.