HERSCHEL/HIFI SEARCH FOR HO AND HO IN IRC+10216: CONSTRAINTS ON MODELS FOR THE ORIGIN OF WATER VAPOR*

We report the results of a sensitive search for the minor isotopologues of water, HO and HO, toward the carbon-rich asymptotic giant branch star IRC+10216 (a.k.a. CW Leonis) using the HIFI instrument on the Herschel Space Observatory. This search was motivated by the fact that any detection of isotopic enhancement in the HO and HO abundances would have strongly implicated CO photodissociation as the source of the atomic oxygen needed to produce water in a carbon-rich circumstellar envelope. Our observations place an upper limit of 1/470 on the HO/HO abundance ratio. Given the isotopic 17O/16O ratio of 1/840 inferred previously for the photosphere of IRC+10216, this result places an upper limit of a factor 1.8 on the extent of any isotope-selective enhancement of HO in the circumstellar material, and provides an important constraint on any model that invokes CO photodissociation as the source of O for H2O production. In the context of the clumpy photodissociation model proposed previously for the origin of water in IRC+10216, our limit implies that 12C16O (not 13C16O or SiO) must be the dominant source of 16O for H2O production, and that the effects of self-shielding can only have reduced the 12C16O photodissociation rate by at most a factor ~2.