[Paper Review] Hydro-, Magnetohydro-, and Dust-Gas Dynamics of Protoplanetary Disks

G. Lesur, B. Ercolano+17 more|arXiv (Cornell University)|Mar 18, 2022

Astrophysics and Star Formation Studies46 citations

TL;DR

This chapter surveys the latest understanding of gas and dust dynamics in protoplanetary disks, focusing on hydrodynamic and magnetohydrodynamic turbulence, dust-gas instabilities, dust clumping, and disk winds, and connects theory to observations.

ABSTRACT

The building of planetary systems is controlled by the gas and dust dynamics of protoplanetary disks. While the gas is simultaneously accreted onto the central star and dissipated away by winds, dust grains aggregate and collapse to form planetesimals and eventually planets. This dust and gas dynamics involves instabilities, turbulence and complex non-linear interactions which ultimately control the observational appearance and the secular evolution of these disks. This chapter is dedicated to the most recent developments in our understanding of the dynamics of gaseous and dusty disks, covering hydrodynamic and magnetohydrodynamic turbulence, gas-dust instabilities, dust clumping and disk winds. We show how these physical processes have been tested from observations and highlight standing questions that should be addressed in the future.

Motivation & Objective

• Explain how angular momentum is transported in protoplanetary disks and the roles of turbulence and winds in driving accretion.
• Describe how dust growth, settling, and drag coupling affect planetesimal formation.
• Summarize thermo-hydrodynamic instabilities (VSI, COS, ZVI) and their impact on disk turbulence and structure.
• Discuss observational constraints and how they test models of gas-dust dynamics and disk winds.

Proposed method

• Review key physical frameworks governing disk dynamics, including accretion theory and angular momentum transport (alpha viscosity, mass-loss parameters).
• Present thermo-hydrodynamic instability criteria using cooling timescales (beta_cool) and buoyancy (N_R^2, N_z^2) to classify instabilities.
• Summarize analytic criteria and stability conditions for VSI, COS, and ZVI and relate them to disk thermodynamics and structure.
• Discuss how dust-gas coupling affects sedimentation, drift, and concentration via Stokes number and diffusion.
• Integrate findings with simulations and observations, highlighting anisotropic turbulence and dust-trapping mechanisms.

Experimental results

Research questions

• RQ1 What are the conditions under which vertical shear instability (VSI), convective overstability (COS), and zombie vortex instability (ZVI) arise in protoplanetary disks?
• RQ2 How do cooling timescales and buoyancy profiles influence the strength and morphology of thermo-hydrodynamic turbulence?
• RQ3 To what extent do hydrodynamic instabilities generate dust concentration, trapping, and potentially planetesimal formation in disks?
• RQ4 How does dust-gas coupling modify angular momentum transport and observable disk properties in the presence of these instabilities?

Key findings

• VSI can produce moderate radial angular momentum transport with alpha_S ~ 1e-4 and strong vertical anisotropy (alpha_z ~ 1e-2).
• COS and SBI can amplify vortices and drive outward angular momentum transport around alpha_S ~ 1e-3, depending on buoyancy and cooling parameters.
• Vortices from COS/SBI persist in 2D/3D simulations and can act as efficient dust traps, aiding planetesimal formation.
• Dust turbulence diffusion (alpha_D) and gas-dust coupling depend on Stokes number and may differ from alpha_S, impacting dust settling and transport.
• VSI remains active in magnetized disks in non-ideal MHD regimes, and dust dynamics under VSI can interact with streaming instabilities and planetesimal growth.