COMPARISON OF THE DUST AND GAS RADIAL STRUCTURE IN THE TRANSITION DISK [PZ99] J160421.7-213028

We present ALMA observations of the 880 m continuum and CO J = 3- 2 line emission from the transition disk around [PZ99] J160421.7- 213028, a solar mass star in the Upper Scorpius OB association. Analysis of the continuum data indicates that 80% of the dust mass is concentrated in an annulus extending between 79 and 114 AU in radius. Dust is robustly detected inside the annulus, at a mass surface density 100 times lower than that at 80 AU. The CO emission in the inner disk also shows a significantly decreased mass surface density, but we infer a cavity radius of only 31 AU for the gas. The large separation of the dust and gas cavity edges, as well as the high radial concentration of millimeter- sized dust grains, is qualitatively consistent with the predictions of pressure trap models that include hydrodynamical disk- planet interactions and dust coagulation/ fragmentation processes.