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We report the results of observations of ten rotational transitions of watervapor toward the carbon-rich AGB (asymptotic giant branch) star IRC+10216 (CWLeonis), carried out with Herschel's HIFI instrument. Each transition wassecurely detected by means of observations using the dual beam switch mode ofHIFI. The measured line ratios imply that water vapor is present in the inneroutflow at small distances (few x 1.E+14 cm) from the star, confirming recentresults reported by Decin et al. from observations with Herschel's PACS andSPIRE instruments. This finding definitively rules out the hypothesis that theobserved water results from the vaporization of small icy objects in circularorbits. The origin of water within the dense C-rich envelope of IRC+10216remains poorly understood. We derive upper limits on the H2-17O/H2-16O andH2-18O/H2-16O isotopic abundance ratios of ~ 5.E-3 (3 sigma), providingadditional constraints on models for the origin of the water vapor inIRC+10216.

Herschel/HIFI observations of IRC+10216: water vapor in the inner envelope of a carbon-rich AGB star