Global drag-induced instabilities in protoplanetary disks

TL;DR

This paper investigates global instabilities caused by drag in protoplanetary disks, revealing mechanisms for planetesimal formation and implications for planetary system development.

Contribution

It introduces a model using the Fokker-Planck equation to identify unstable modes in particle-gas interactions within protoplanetary disks, highlighting new pathways for planetesimal formation.

Findings

Identified two global unstable spiral modes with azimuthal wavenumber m=1.

Particles accumulate exponentially at pressure maxima, leading to planetesimal formation.

Implications for formation of asteroid belts, Kuiper belt objects, and planetary cores.

Abstract

We use the Fokker-Planck equation and model the dispersive dynamics of solid particles in annular protoplanetary disks whose gas component is more massive than the particle phase. We model particle--gas interactions as hard sphere collisions, determine the functional form of diffusion coefficients, and show the existence of two global unstable modes in the particle phase. These modes have spiral patterns with the azimuthal wavenumber $m=1$ and rotate slowly. We show that in ring-shaped disks, the phase space density of solid particles increases linearly in time towards an accumulation point near the location of pressure maximum, while instabilities grow exponentially. Therefore, planetesimals and planetary cores can be efficiently produced near the peaks of unstable density waves. In this mechanism, particles migrating towards the accumulation point will not participate in the formation of planets, and should eventually form a debris ring like the main asteroid belt or classical Kuiper belt objects. We present the implications of global instabilities to the formation of ice giants and terrestrial planets in the solar system.