A SIGNIFICANTLY LOW CO ABUNDANCE TOWARD THE TW Hya PROTOPLANETARY DISK: A PATH TO ACTIVE CARBON CHEMISTRY?

In this Letter we report the CO abundance relative to H 2 derived toward the circumstellar disk of the T-Tauri star TW Hya from the HD (1 − 0) and C 18 O (2 − 1) emission lines. The HD (1 − 0) line was observed by the Herschel Space Observatory Photodetector Array Camera and Spectrometer whereas C 18 O (2 − 1) observations were carried out with the Submillimeter Array at a spatial resolution of 2 8 × 1 9 (corresponding to ∼151 × 103 AU). In the disk's warm molecular layer ( T > 20 K) we measure a disk-averaged gas-phase CO abundance relative to H 2 of χ(CO) = (0.1–3) × 10 −5 , substantially lower than the canonical value of χ(CO) = 10 −4 . We infer that the best explanation of this low χ(CO) is the chemical destruction of CO followed by rapid formation of carbon chains, or perhaps CO 2 , that can subsequently freeze-out, resulting in the bulk mass of carbon locked up in ice grain mantles and oxygen in water. As a consequence of this likely time-dependent carbon sink mechanism, CO may be an unreliable tracer of H 2 gas mass.