Tsukagoshi et al
We report ∼3 au resolution imaging observations of the protoplanetary disk around TW Hya at 138 and 230 GHz with the Atacama Large Millimeter/Submillimeter Array. Our observations revealed two deep gaps (∼25--50 %) at 22 and 37 au and shallower gaps (a few %) at 6, 28, and 44 au, as recently reported by Andrews et al. (2016). The central hole with a radius of ∼3 au was also marginally resolved. The most remarkable finding is that the power-law index of the dust opacity β, derived from the spectral index α between bands 4 and 6, peaks at the 22 au gap with β∼1.7 and decreases toward the disk center to β∼0. Our model fitting suggests that the overall disk structure can be reproduced with the inner hole and the gaps at 22 and 37 au. 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 ≲1.5 MNeptune although other origins may be possible. The planet-induced gap is supported by the fact that β is enhanced at the 22 au gap, indicating a deficit of ∼mm-sized grains within the gap due to dust filtration by a planet. Alternatively, the destruction of large dust aggregates due to the sintering of major volatiles might cause the multiple ring structure. We also find weak sinusoidal patterns with wavelengths of 5--10 au, which may be related to dynamical instabilities within the disk.