WFPC2 images of a face-on disk surrounding TW Hydrae

Hubble Space Telescope observations of the isolated T Tauri star TW Hydrae reveal the presence of a compact circumstellar nebula. After subtraction of a reference point-spread function (PSF), a smooth, symmetrical, circular h alo can be seen in both R- and I-band WFPC2 images. Its intensity declines with radius until reaching an outer sensitivity limit at 3 ".5 (approximate to 200 AU). Numerical experiments show that PSF subtraction artifacts cann ot account for the halo's brightness distribution. Instead, the most likely explanation is that the halo is a face-on circumstellar disk. The radial b rightness profile of the halo is complex and can be described with multiple , contiguous zones with individual power-law intensity relations. The halo appears slightly blue relative to the star, especially in the outer zones. We compare the TW Hya halo to single-scattering models of face-on disks wit h multiple radial zones. While optically thin disk models with vertical opt ical depth tau(upsilon) approximate to 10(-2) can reproduce the relative br ightness of the nebula and star, we find that such models have large midpla ne optical depths and are therefore not self-consistent. We present an opti cally thick disk model that matches the radial brightness profile self-cons istently and has a dust mass close to that implied by submillimeter continu um measurements. The zonal structure found in the disk could arise from rad ial variations in the dust properties that determine the local equilibrium temperature or perhaps via dynamical effects of unseen companions.