Kastner, Joel H.Qi, ChunhuaGorti, UmaHily-Blant, PierreOberg, KarinForveille, ThierryAndrews, SeanWilner, David2016-01-212015Kastner, Joel H., Chunhua Qi, Uma Gorti, Pierre Hily-Blant, Karin Oberg, Thierry Forveille, Sean Andrews, and David Wilner. 2015. “A Ring of C2H in the Molecular Disk Orbiting TW Hya.” The Astrophysical Journal 806 (1) (June 9): 75. doi:10.1088/0004-637x/806/1/75.1538-4357http://nrs.harvard.edu/urn-3:HUL.InstRepos:24820071We have used the Submillimeter Array to image, at ~1farcs5 resolution, C2H $N=3\to 2$ emission from the circumstellar disk orbiting the nearby (D = 54 pc), ~8 Myr-old, ~0.8 ${{M}_{\odot }}$ 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 ($T\gtrsim 40$ 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.en-USJournal Article2016-01-09Kastner, J.H., Qi, C., Gorti, U. Hily-Blant, P., Oberg, K.I., Forveille, T., Andrews, S.M., Wilner, D.2016-01-2110.1088/0004-637x/806/1/75