Evidence for a Developing Gap in a 10 Myr Old Protoplanetary Disk

We have developed a physically self-consistent model of the disk around the nearby 10 Myr old star TW Hya that matches the observed spectral energy distribution and 7 mm images of the disk. The model requires both significant dust-size evolution and a partially evacuated inner disk region, as predicted by theories of planet formation. The outer disk, which extends to at least 140 AU in radius, is very optically thick at infrared wavelengths and quite massive (~0.06 M_☉) for the relatively advanced age of this T Tauri star. This implies long viscous and dust evolution timescales, although dust must have grown to sizes of the order of ~1 cm to explain the submillimeter and millimeter spectral slopes. In contrast, the negligible near-infrared excess emission of this system requires that the disk be optically thin inside ≲4 AU. This inner region cannot be completely evacuated; we need ~0.5 lunar mass of ~1 μm particles remaining to produce the observed 10 μm silicate emission. Our model requires a distinct transition in disk properties at ~4 AU separating the inner and outer disks. The inner edge of the optically thick outer disk must be heated almost frontally by the star to account for the excess flux at mid-infrared wavelengths. We speculate that this truncation of the outer disk may be the signpost of a developing gap due to the effects of a growing protoplanet; the gap is still presumably evolving because material still resides in it, as indicated by the silicate emission, the molecular hydrogen emission, and the continued accretion onto the central star (albeit at a much lower rate than typical of younger T Tauri stars). Thus, TW Hya may become the Rosetta stone for our understanding of the evolution and dissipation of protoplanetary disks.