Resolving Planet Formation through Observations of Dust in Disks at Low Optical Depths
In a few years, ALMA has revolutionized our view of protoplanetary disk structure and evolution. Its resolution and sensitivity have unveiled a wide variety of possibly related phenomena such as azimuthal assymmetries, radial flows, ringed structure, and spiral features. However, the innermost tens of astronomical units in most disks are opaque even at the shortests wavelengths (~3 mm) currently observable with ALMA. Recent observations with the VLA in Q-band (~ 7 mm) have provided the first detailed views of the inner regions of disks in the optically-thin regime. Arguably, these observations make one of the strongest cases for ALMA Band 1: sensitive mapping from 6 to 8.5 mm at the angular resolution that only ALMA can give is necessary to detect and resolve the phenomenology associated with the formation of terrestrial planets.