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We have modeled the emission of H2O rotational lines from the extreme C-richstar IRC+10216. Our treatment of the excitation of H2O emissions takes intoaccount the excitation of H2O both through collisions, and through the pumpingof the nu2 and nu3 vibrational states by dust emission and subsequent decay tothe ground state. Regardless of the spatial distribution of the watermolecules, the H2O 1_{10}-1_{01} line at 557 GHz observed by the SubmillimeterWave Astronomy Satellite (SWAS) is found to be pumped primarily through theabsorption of dust-emitted photons at 6 $\mu$m in the nu2 band. As noted byprevious authors, the inclusion of radiative pumping lowers the ortho-H2Oabundance required to account for the 557 GHz emission, which is found to be(0.5-1)x10^{-7} if the presence of H2O is a consequence of vaporization oforbiting comets or Fischer-Tropsch catalysis. Predictions for othersubmillimeter H2O lines that can be observed by the Herschel Space Observatory(HSO) are reported. Multitransition HSO observations promise to reveal thespatial distribution of the circumstellar water vapor, discriminating among theseveral hypotheses that have been proposed for the origin of the H2O vapor inthe envelope of IRC+10216. We also show that, for observations with HSO, theH2O 1_{10}-1_{01} 557 GHz line affords the greatest sensitivity in searchingfor H2O in other C-rich AGB stars.

Water vapor emission from IRC+10216 and other carbon-rich stars: model predictions and prospects for multitransition observations