Herschel/HIFI search for H2-17O and H2-18O 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, 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.

14 pages, 4 figures, accepted for publication in ApJ Letters

References in corpus (5)

• Oxygen Chemistry in the Circumstellar Envelope of the Carbon-Rich Star IRC+10216
• Warm water vapour in the sooty outflow from a luminous carbon star
• Water in IRC+10216: a genuine formation process by shock-induced chemistry in the inner wind
• Herschel/HIFI observation of highly excited rotational lines of HNC toward IRC +10 216
• Water vapor emission from IRC+10216 and other carbon-rich stars: model predictions and prospects for multitransition observations

Cited by in corpus (7)

• ExoMol molecular line lists XIX: high accuracy computed hot line lists for H${}_2$${}^{18}$O and H${}_2$${}^{17}$O
• A 1.3 cm line survey toward IRC +10216
• Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars
• Carbon chemistry in IRC+10216: Infrared detection of diacetylene
• Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216
• Chemistry in AGB stars: successes and challenges
• Condensation sequence of circumstellar cluster seeds (CSCCS)