We have used the Submillimeter Array to image, at ∼1.″5 resolution, C2H N=3\to 2 emission from the circumstellar disk orbiting the nearby (D = 54 pc), ∼8 Myr-old, ∼0.8 {{M}ȯ } classical T Tauri star TW Hya. The SMA imaging reveals that the C2H emission exhibits a ring-like morphology. Based on a model in which the C2H column density follows a truncated radial power-law distribution, we find that the inner edge of the ring lies at ∼45 AU, and that the ring extends to at least ∼120 AU. Comparison with previous (single-dish) observations of C2H N=4\to 3 emission indicates that the C2H molecules are subthermally excited and, hence, that the emission arises from the relatively warm (T≳ 40 K), tenuous (n\ll {{10}7} cm‑3) upper atmosphere of the disk. Based on these results and comparisons of the SMA C2H map with previous submillimeter and scattered-light imaging, we propose that the C2H emission most likely traces particularly efficient photo-destruction of small grains and/or photodesorption and photodissociation of hydrocarbons derived from grain ice mantles in the surface layers of the outer disk. The presence of a C2H ring in the TW Hya disk hence likely serves as a marker of dust grain processing and radial and vertical grain size segregation within the disk.
