TY - JOUR AU - Neufeld, D.A. AU - Tolls, V. AU - Agúndez, M. AU - González-Alfonso, E. AU - Decin, L. AU - Daniel, F. AU - Cernicharo, J. AU - Melnick, G.J. AU - Schmidt, M. AU - Szczerba, R. T1 - Herschel/HIFI search for H217O and H2 18O in IRC+10216: Constraints on models for the origin of water VAPOR LA - eng PY - 2013/04/10/ T2 - Astrophysical Journal Letters SN - 2041-8205 VL - 767 IS - 1 AB - We report the results of a sensitive search for the minor isotopologues of water, H172 O and H182 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 H17 2 O and H182 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 H172 O/H16 2 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 172 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. DO - 10.1088/2041-8205/767/1/L3 UR - https://portalcientifico.uah.es/documentos/614e79f568c341284de7cbd9 DP - Dialnet - Portal de la Investigación ER -