Herschel/HIFI search for H2-17O and H2-18O 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, H2-17O and H2-18O, 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 H2-17O and H2-18O 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 H2-17O/H2-16O 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 H2-17O 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 12C-16O (not 13C-16O or SiO) must be the dominant source of 16O for H2O
production, and that the effects of self-shielding can only have reduced the
12C-16O photodissociation rate by at most a factor ~ 2.
