2015ApJ...812..126C

A pathway to the formation of planetesimals, and eventually giant planets, may occur in concentrations of dust grains trapped in pressure maxima. Dramatic crescent-shaped dust concentrations have been seen in recent radio images at submillimeter wavelengths. These disk asymmetries could represent the initial phases of planet formation in the dust trap scenario, provided that grain sizes are spatially segregated. A testable prediction of azimuthal dust trapping is that progressively larger grains should be more sharply confined and should follow a distribution that is markedly different from the gas. However, gas tracers such as ¹²CO and the infrared emission from small grains are both very optically thick where the submillimeter continuum originates, so previous observations have been unable to test the trapping predictions or to identify compact concentrations of larger grains required for planet formation by core accretion. Here we report multifrequency observations of HD 142527, from 34 to 700 GHz, that reveal a compact concentration of grains approaching centimeter sizes, with a few Earth masses, embedded in a large-scale crescent of smaller, submillimeter-sized particles. The emission peaks at wavelengths shorter than ∼1 mm are optically thick and trace the temperature structure resulting from shadows cast by the inner regions. Given this temperature structure, we infer that the largest dust grains are concentrated in the 34 GHz clump. We conclude that dust trapping is efficient enough for grains observable at centimeter wavelengths to lead to compact concentrations.