HERSCHEL/HIFI SEARCH FOR H217O AND H218O 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, (H2O)-O-17 and (H2O)-O-18, 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 (H2O)-O-17 and (H2O)-O-18 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 (H2O)-O-17 and (H2O)-O-16 abundance ratio. Given the isotopic O-17/O-16 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 (H2O)-O-17 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 (CO)-C-12-O-16 ( not (CO)-C-13-O-16 or SiO) must be the dominant source of O-16 for H2O production, and that the effects of self-shielding can only have reduced the (CO)-C-12-O-16 photodissociation rate by at most a factor similar to 2.