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 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.″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 H2 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 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 H 2 gas mass. © 2013. The American Astronomical Society. All rights reserved.