2013ApJ...776L..38F

In this Letter we report the CO abundance relative to H₂ derived toward the circumstellar 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 Space Observatory Photodetector Array Camera and Spectrometer whereas C¹⁸O (2 - 1) observations were carried out with the Submillimeter Array at a spatial resolution of 2.''8x1.''9 (corresponding to ∼151x103 AU). In the disk's warm molecular layer (T > 20 K) we measure a disk-averaged gas-phase CO abundance relative to H₂ of χ(CO) = (0.1-3)x10^–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 CO₂, 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 H₂ gas mass.