Surface accretion as a dust retention mechanism in protoplanetary disks. I. Formulation and proof-of-concept simulations.

This article investigates a dust retention mechanism in protoplanetary disks driven by magnetohydrodynamical (MHD) wind-induced surface gas accretion flows. It addresses the challenge that dust grains in these disks are observed to be smaller and less sticky than previously assumed, resulting in slower radial drift and complicating planetesimal formation via traditional dust coagulation and concentration pathways. The proposed mechanism relies on gas removal at the disk surface by MHD winds while slowly drifting, settled dust near the midplane remains, leading to an enhanced midplane dust-to-gas mass ratio. One-dimensional simulations incorporating dust growth, fragmentation, and radial transport confirm that when the dust radial drift timescale exceeds the gas removal timescale—particularly for poorly sticky grains with low sticking threshold velocities (~0.1–1 m/s)—the dust-to-gas ratio can increase above unity, potentially enabling planetesimal formation via streaming and gravitational instabilities. The study highlights that this retention is sensitive to the vertical structure of gas accretion flows and turbulence parameters, and suggests further work to integrate detailed MHD flow models and dust-gas feedback effects.