A GAP WITH A DEFICIT OF LARGE GRAINS IN THE PROTOPLANETARY DISK AROUND TW Hya

We report similar to 3 au resolution imaging observations of the protoplanetary disk around TW Hya at 145 and 233 GHz with the Atacama Large Millimeter/submillimeter Array. Our observations revealed two deep gaps (similar to 25%-50%) at 22 and 37 au and shallower gaps (a few percent) at 6, 28, and 44 au, as recently reported by Andrews et al. The central hole with a radius of similar to 3 au was also marginally resolved. The most remarkable finding is that the spectral index alpha(R) between bands 4 and 6 peaks at the 22 au gap. The derived power-law index of the dust opacity b(R) is similar to 1.7 at the 22 au gap and decreases toward the disk center to similar to 0. The most prominent gap at 22 au could be caused by the gravitational interaction between the disk and an unseen planet with a mass of less than or similar to 1.5 M-Neptune, although other origins may be possible. The planet-induced gap is supported by the fact that beta(R) is enhanced at the 22 au gap, indicating a deficit of similar to millimeter-sized grains within the gap due to dust filtration by a planet.