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

Abstract

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 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 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 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 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 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.