2015ApJ...806...75K

We have used the Submillimeter Array to image, at Å1."5 resolution, C₂ H emission from the circumstellar disk orbiting the nearby (D = 54 pc), Å8 Myr-old, Å0.8 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 Å45 AU, and that the ring extends to at least Å120 AU. Comparison with previous (single-dish) observations of C₂H emission indicates that the C₂H molecules are subthermally excited and, hence, that the emission arises from the relatively warm ( K), tenuous (/cm3) 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 C₂ H 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.