Abstract

Recent progress suggests that the accretion of rocky cores could occur efficiently in the outer regions of protoplanetary disks hosting large gaps, at stellocentric radii comparable to the Kuiper belt in the solar system. Such annular cavities are thought to be sculpted by massive giant planets, in a transition phase when gas reservoirs are exhausted or dissipated. During this dynamical clearing, azimuthal overdensities at the outer edge of the gaps trap dust grains, and segregate the gas. The dust concentrations show azimuthal asymmetry, resembling a horseshoe as seen in recent sub-mm images. The aerodynamic coupling of dust grains with pressure gradients predicts that larger grains are trapped more efficiently in such horseshoes, leading to grain growth, formation of planetesimals, and eventually rocky cores. Here we bring observational support for efficient grain growth in a dust trap at large stellocentric radii. Opaque clumps at 1.2mm in the outer disk of HD142527 display the flattest opacity laws, as expected for the larger grains. At the observable extreme of the size distribution, we find cm-wavelength clumps coincident with the 1.2mm opacity maxima. Interestingly the temperature in these rocky clumps are <25K, and they also coincide with decrements in the molecular emission of volatile species. These cm-wavelength emission maxima probably correspond to clumps of large grains with icy mantles, as expected in the context of rocky core accretion in outer disks.

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