ALMA unveils rings and gaps in the protoplanetary system

The protoplanetary system HD 169142 is one of the few cases where a potential candidate protoplanet has recently been detected by direct imaging in the near-infrared. To study the interaction between the protoplanet and the disk itself, observations of the gas and dust surface density structure are needed. This paper reports new ALMA observations of the dust continuum at 1.3 mm, 12CO, 13CO, and C18O J = 2−1 emission from the system HD 169142 (which is observed almost face-on) at an angular resolution of ~\hbox{$0\farcs3 \,\times\, 0\farcs2$} (~35 × 20 au). The dust continuum emission reveals a double-ring structure with an inner ring between \hbox{$0\farcs17{-}0\farcs28$} (~20−35 au) and an outer ring between \hbox{$0\farcs48{-}0\farcs64$} (~56−83 au). The size and position of the inner ring is in good agreement with previous polarimetric observations in the near-infrared and is consistent with dust trapping by a massive planet. No dust emission is detected inside the inner dust cavity (R ≲ 20 au) or within the dust gap (~35−56 au) down to the noise level. In contrast, the channel maps of the J = 2−1 line of the three CO isotopologs reveal gas inside the dust cavity and dust gap. The gaseous disk is also much larger than the compact dust emission; it extends to ~1\hbox{$\farcs5$} (~180 au) in radius. This difference and the sharp drop of the continuum emission at large radii point to radial drift of large dust grains (>μm size). Using the thermo-chemical disk code dal