MWC 758 Seems to be Forming Planetesimals

Compact dust concentration in the MWC 758 protoplanetary disk

Authors:

Marino et al

Abstract:

The formation of planetesimals requires that primordial dust grains grow from micron- to km-sized bodies. Dust traps caused by gas pressure maxima have been proposed as regions where grains can concentrate and grow fast enough to form planetesimals, before radially migrating onto the star. We report new VLA Ka & Ku observations of the protoplanetary disk around the Herbig Ae/Be star MWC 758. The Ka image shows a compact emission region in the outer disk indicating a strong concentration of big dust grains. Tracing smaller grains, archival ALMA data in band 7 continuum shows extended disk emission with an intensity maximum to the north-west of the central star, which matches the VLA clump position. This segregation of grains sizes is expected in the context of dust trapping, where big grains are trapped more easily than smaller grains in gas pressure maxima. We develop a non-axisymmetric parametric model inspired by a steady state vortex solution which reproduces the observations, including the spectral energy distribution. Finally, we compare the radio continuum with SPHERE scattered light data. The ALMA continuum spatially coincides with a region devoid of scattered polarised emission and the VLA clump is offset to the north of the north-western spiral-like feature, indicating moderate or no flaring in the outer disk.