2007ApJ...669..412G

We have modeled the emission of H₂ O rotational lines from the extreme C-rich star IRC +10216. Our treatment of the excitation of H₂ O emissions takes into account the excitation of H₂ O both through collisions and through the pumping of the ν₂ and ν₃vibrational states by dust emission and subsequent decay to the ground state. Regardless of the spatial distribution of the water molecules, the H₂O(1₁₀-1₀₁) line at 557 GHz observed by the Submillimeter Wave Astronomy Satellite (SWAS) is found to be pumped primarily through the absorption of dust-emitted photons at 6 µm in the ν₂band. As noted by previous authors, the inclusion of radiative pumping lowers the ortho-H₂ O abundance 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 of orbiting comets or Fischer-Tropsch catalysis. Predictions for other submillimeter H₂ O lines that can be observed by the Herschel Space Observatory (HSO) are reported. Multitransition HSO observations promise to reveal the spatial distribution of the circumstellar water vapor, discriminating among the several hypotheses that have been proposed for the origin of the H₂ O vapor in the envelope of IRC +10216. We also show that, for observations with HSO, the H₂O(1₁₀-1₀₁) 557 GHz line affords the greatest sensitivity in searching for H₂ O in other C-rich AGB stars.