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 sub(2) derived toward the circumstellar disk of the T-Tauri star TW Hya from the HD (1 - 0) and C super(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 super(18)O (2 - 1) observations were carried out with the Submillimeter Array at a spatial resolution of 2''.8 x 1''.9 (corresponding to ~151 x 103 AU). In the disk's warm molecular layer (T > 20 K) we measure a disk-averaged gas-phase CO abundance relative to H sub(2) of chi (CO) = (0.1-3) x 10 super(-5), substantially lower than the canonical value of chi (CO) = 10 super(-4). We infer that the best explanation of this low chi (CO) is the chemical destruction of CO followed by rapid formation of carbon chains, or perhaps CO sub(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 sub(2) gas mass.