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


Cecile Favre, L. Ilsedore Cleeves, Edwin A. Bergin, Chunhua Qi, Geoffrey A. Blake (Submitted on 20 Sep 2013)

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" x 1.9" (corresponding to 142 x 97 AU). In the disk's warm molecular layer (T>20 K) we measure a disk-averaged gas-phase CO abundance relative to H2 of $\chi{\rm(CO)}=(0.1-3)x10^{-5}$, substantially lower than the canonical value of $\chi{\rm(CO)}=10^{-4}$. 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 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.

Comments: Accepted for publication in the Astrophysical Journal Letters (ApJL). 9 pages, 4 figures, 1 table

Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:1309.5370 [astro-ph.SR] (or arXiv:1309.5370v1 [astro-ph.SR] for this version) Submission history From: Cecile Favre [v1] Fri, 20 Sep 2013 20:10:52 GMT (1570kb)

 

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