We report the results of a sensitive search for the minor isotopologues of water , H _ { 2 } ^ { 17 } O and H _ { 2 } ^ { 18 } O , toward the carbon-rich AGB 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 H _ { 2 } ^ { 17 } O and H _ { 2 } ^ { 18 } O 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 H _ { 2 } ^ { 17 } O/H _ { 2 } ^ { 16 } O abundance ratio .
Given the isotopic ^ { 17 } O/ ^ { 16 } O 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 H _ { 2 } ^ { 17 } O in the circumstellar material , and provides an important constraint on any model that invokes CO photodissociation as the source of O for H _ { 2 } O production .
In the context of the clumpy photodissociation model proposed previously for the origin of water in IRC+10216 , our limit implies that ^ { 12 } C ^ { 16 } O ( not ^ { 13 } C ^ { 16 } O or SiO ) must be the dominant source of ^ { 16 } O for H _ { 2 } O production , and that the effects of self-shielding can only have reduced the ^ { 12 } C ^ { 16 } O photodissociation rate by at most a factor \sim 2 .