NONLOCAL RADIATIVE-TRANSFER FOR MOLECULES - MODELING POPULATION INVERSIONS IN WATER MASERS

A new method is developed using accelerated lambda iteration (ALI) tec hniques (Jones et al.; Randell et al.) to compute H2O level population s for energy levels up to 10(4) K, in a one-dimensional plane-parallel slab geometry. This represents an advance of general significance in the interpretation of line data for many optically thick molecular spe cies, masing or otherwise, through the development both of exact metho ds of radiative transfer and of methods which allow specific structure to be introduced into the molecular environment. Trial calculations a re presented, using the most recent rate coefficients for rotationally inelastic collisions, covering physical conditions for a wide range o f molecular hydrogen number density n(H-2), kinetic temperature T-K, d ust temperature T, and H2O number density in an essentially uniform me dium, including conditions which may be encountered in both shocked st ar-forming regions and the circumstellar envelopes of late-type stars. For a range of conditions involving n (H-2) between 10(8) and 10(10) cm(-3), T-K = 200 to 2000 K, T-d = 3 to 600 K, large inversions are fo und in all known lines observed to show strong maser action. In common with results of earlier studies, 22-, 321-, 183- and 325-GHz lines ar e found to be collisionally pumped. The strong observed 437-, 439- and 471-GHz lines are, however, found to have an important radiative pump ing component. Our results show for the first time how the 437-GHz lin e, the strongest line in U Her, may in principle become inverted. Acco rding to the present calculations, a number of new maser transitions, observable from the ground, may prove detectable in ortho H2O transiti ons at 443, 504, 646 and 864 GHz, and in para H2O transitions at 488, 505, 646, 863 and 906 GHz. In addition, a group of lines, with frequen cies > 1000 GHz, are predicted to show significant inversions, and the se maser emissions will be sought in observations with the Infrared Sp ace Observatory.