Herschel/HIFI search for H_2 ~(17)O and H_2 ~(18)O in IRC+10216: Constraints on models for the origin of water VAPOR

摘要

We report the results of a sensitive search for the minor isotopologues of water, H~(17) _2 O and H~(18) _2 O, toward the carbon-rich asymptotic giant branch star IRC+10216 (a.k.a. CWLeonis) 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~(17) _2 O and H~(18) _2 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~(17) _2 O/H16 2 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 ~(17) _2 O in the circumstellar material, and provides an important constraint on any model that invokes CO photodissociation as the source of O for H_2O 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_2O production, and that the effects of self-shielding can only have reduced the ~(12)C~(16)O photodissociation rate by at most a factor ～2.