THE TW Hya DISK AT 870 μm: COMPARISON OF CO AND DUST RADIAL STRUCTURES

Abstract

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