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

Abstract

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.

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