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In this Letter we report the CO abundance relative to H2 derived toward thecircumstellar disk of the T-Tauri star TW Hya from the HD (1-0) and C18O (2-1)emission lines. The HD (1-0) line was observed by the Herschel SpaceObservatory Photodetector Array Camera and Spectrometer whereas C18O (2-1)observations were carried out with the Submillimeter Array at a spatialresolution of 2.8" x 1.9" (corresponding to 142 x 97 AU). In the disk's warmmolecular layer (T>20 K) we measure a disk-averaged gas-phase CO abundancerelative to H2 of $\chi{\rm(CO)}=(0.1-3)x10^{-5}$, substantially lower than thecanonical value of $\chi{\rm(CO)}=10^{-4}$. We infer that the best explanationof this low $\chi$(CO) is the chemical destruction of CO followed by rapidformation of carbon chains, or perhaps CO2, that can subsequently freeze-out,resulting in the bulk mass of carbon locked up in ice grain mantles and oxygenin water. As a consequence of this likely time-dependent carbon sink mechanism,CO may be an unreliable tracer of H2 gas mass.

A Significantly Low CO Abundance Toward the TW Hya Protoplanetary Disk: A Path to Active Carbon Chemistry?