The TW Hya Disk at 870 microns: Comparison of CO and Dust Radial Structures

TL;DR

This study uses high-resolution submillimeter observations to compare the radial structures of dust and CO gas in the TW Hya disk, revealing a sharp dust edge at 60 AU and extended CO emission, indicating complex disk substructures.

Contribution

It provides the first detailed comparison of dust and CO radial distributions in TW Hya, highlighting a dust edge inconsistent with simple models and suggesting disk substructure or grain evolution.

Findings

Dust has a sharp outer edge near 60 AU.

CO emission extends to at least 215 AU.

Simple models cannot simultaneously fit dust and CO data.

Abstract

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.