  We present high resolution (0.3" = 16 AU), high signal-to-noise ratio
Submillimeter Array observations of the 870 microns (345 GHz) continuum and CO
J=3--2 line emission from the protoplanetary disk around TW Hya. Using
continuum and line radiative transfer calculations, those data and the
multiwavelength spectral energy distribution are analyzed together in the
context of simple two-dimensional parametric disk structure models. Under the
assumptions of a radially invariant dust population and (vertically integrated)
gas-to-dust mass ratio, we are unable to simultaneously reproduce the CO and
dust observations with model structures that employ either a single, distinct
outer boundary or a smooth (exponential) taper at large radii. Instead, we find
that the distribution of millimeter-sized dust grains in the TW Hya disk has a
relatively sharp edge near 60 AU, contrary to the CO emission (and
optical/infrared scattered light) that extends to a much larger radius of at
least 215 AU. We discuss some possible explanations for the observed radial
distribution of millimeter-sized dust grains and the apparent CO-dust size
discrepancy, and suggest that they may be hallmarks of substructure in the dust
disk or natural signatures of the growth and radial drift of solids that might
be expected for disks around older pre-main sequence stars like TW Hya.
