  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), ~8
Myr-old, ~0.8 Msun 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 (4-3)
emission indicates that the C2H molecules are subthermally excited and, hence,
that the emission arises from the relatively warm, tenuous upper atmosphere of
the disk. 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.

Solar and Stellar Astrophysics