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We have used the Submillimeter Array to image, at ~1.5" resolution, C2H (3-2)emission from the circumstellar disk orbiting the nearby (D = 54 pc), ~8Myr-old, ~0.8 Msun classical T Tauri star TW Hya. The SMA imaging reveals thatthe C2H emission exhibits a ring-like morphology. Based on a model in which theC2H column density follows a truncated radial power-law distribution, we findthat the inner edge of the ring lies at ~45 AU, and that the ring extends to atleast ~120 AU. Comparison with previous (single-dish) observations of C2H (4-3)emission indicates that the C2H molecules are subthermally excited and, hence,that the emission arises from the relatively warm, tenuous upper atmosphere ofthe disk. We propose that the C2H emission most likely traces particularlyefficient photo-destruction of small grains and/or photodesorption andphotodissociation of hydrocarbons derived from grain ice mantles in the surfacelayers of the outer disk. The presence of a C2H ring in the TW Hya disk hencelikely serves as a marker of dust grain processing and radial and verticalgrain size segregation within the disk.

A Ring of C2H in the Molecular Disk Orbiting TW Hya