Compact dust concentration in the MWC 758 protoplanetary disk

TL;DR

This study uses VLA and ALMA observations to reveal dust trapping in the MWC 758 protoplanetary disk, showing big dust grains concentrated in a compact region consistent with a vortex, aiding planetesimal formation.

Contribution

It introduces a non-axisymmetric vortex model to explain dust concentration patterns and compares multi-wavelength data to understand dust segregation in the disk.

Findings

Detection of a compact big dust grain concentration with VLA

Extended smaller grain emission aligned with ALMA data

Spatial correlation between dust traps and disk features

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. The compactness of the Ka emission is expected in the context of dust trapping, as 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 with parameters adequately selected to reproduce 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 spiral-like feature seen in scattered light, while the VLA clump is offset from the scattered light maximum. Moreover, the ALMA map shows a decrement that matches a region devoid of scattered polarised emission. Continuum observations at a different wavelength are necessary to conclude if the VLA-ALMA difference is an opacity or a real dust segregation.