A Gap with a Deficit of Large Grains in the protoplanetary disk around TW Hya

TL;DR

This study uses high-resolution ALMA imaging to reveal multiple gaps in the TW Hya protoplanetary disk, with the most prominent at 22 au likely caused by an unseen planet filtering dust and shaping the disk structure.

Contribution

First high-resolution imaging of TW Hya's disk showing detailed gap structures and spectral index variations, suggesting planet-disk interactions as the cause.

Findings

Deep gaps at 22 and 37 au with significant dust depletion.

Spectral index peaks at the 22 au gap indicating dust filtration.

Possible presence of an unseen Neptune-mass planet causing the prominent gap.

Abstract

We report $\sim$3 au resolution imaging observations of the protoplanetary disk around TW Hya at 145 and 233 GHz with the Atacama Large Millimeter/Submillimeter Array. Our observations revealed two deep gaps ($\sim$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 $\sim3$~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 $\beta (R)$ is $\sim1.7$ at the 22~au gap and decreases toward the disk center to $\sim0$. 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 $\lesssim$1.5 $M_\mathrm{Neptune}$ 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 $\sim$mm-sized grains within the gap due to dust filtration by a planet.