Infrared Views of the TW Hydra Disk

The face-on disk around TW Hya is imaged in scattered light at 1.1 and 1.6 μm using the coronagraph in the Near Infrared Camera and Multi Object Spectrometer aboard the Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20 to 230 AU. The surface brightness declines as a power law of r^-2.6+/-0.1 between 45 and 150 AU. The scattering profile indicates that the disk is flared, not geometrically flat. The disk, while spatially unresolved in thermal radiation at 12 and 18 μm in observations from the W. M. Keck Observatory, shows amorphous and crystalline silicate emission in its spectrum. A disk with silicate grains of a radius ~1 μm in size in its surface layers can explain the color of the scattered light and the shape of the mid-infrared spectrum. Much larger grains in the disk interior are necessary to fit the millimeter-wave spectral energy distribution, and hence grain growth from an original interstellar size population may have occurred.