Non-local radiative transfer for molecules: Modelling population inversions in water masers

A new method is developed using accelerated lambda iteration (ALI) techniques (Jones et al.; Randell et al.) to compute H 2O level populations for energy levels up to 10 4K, 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 species, masing or otherwise, through the development both of exact methods of radiative transfer and of methods which allow specific structure to be introduced into the molecular environment. Trial calculations are presented, using the most recent rate coefficients for rotationally inelastic collisions, covering physical conditions for a wide range of molecular hydrogen number density n(H 2), kinetic temperature T K, dust temperature T d and H 2O number density in an essentially uniform medium, including conditions which may be encountered in both shocked star-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 found in all known lines observed to show strong maser action. In common with results of earlier studies, 22-, 321-, 183- and 325-GHz lines are found to be collisionally pumped. The strong observed 437-, 439- and 471-GHz lines are, however, found to have an important radiative pumping component. Our results show for the first time how the 437-GHz line, the strongest line in U Her, may in principle become inverted. According to the present calculations, a number of new maser transitions, observable from the ground, may prove detectable in ortho H 2O transitions at 443, 504, 646 and 864 GHz, and in para H 2O transitions at 488, 505, 646, 863 and 906 GHz. In addition, a group of lines, with frequencies > 1000 GHz, are predicted to show significant inversions, and these maser emissions will be sought in observations with the Infrared Space Observatory. © 1997 RAS.