We have used the Submillimeter Array to image , at \sim 1.5 ^ { \prime \prime } resolution , C _ { 2 } H N = 3 \rightarrow 2 emission from the circumstellar disk orbiting the nearby ( D = 54 pc ) , \sim 8 Myr-old , \sim 0.8 M _ { \odot } classical T Tauri star TW Hya .
The SMA imaging reveals that the C _ { 2 } H emission exhibits a ring-like morphology .
Based on a model in which the C _ { 2 } H column density follows a truncated radial power-law distribution , we find that the inner edge of the ring lies at \sim 45 AU , and that the ring extends to at least \sim 120 AU .
Comparison with previous ( single-dish ) observations of C _ { 2 } H N = 4 \rightarrow 3 emission indicates that the C _ { 2 } H molecules are subthermally excited and , hence , that the emission arises from the relatively warm ( T \stackrel { > } { \sim } 40 K ) , tenuous ( n < < 10 ^ { 7 } cm { { } ^ { -3 } } ) upper atmosphere of the disk .
Based on these results and comparisons of the SMA C _ { 2 } H map with previous submm and scattered-light imaging , we propose that the C _ { 2 } H 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 C _ { 2 } 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 .