Dust Coagulation Reconciles Protoplanetary Disk Observations with the Vertical Shear Instability. I. Dust Coagulation and the VSI Dead Zone

TL;DR

This paper shows that dust coagulation in protoplanetary disks can suppress the Vertical Shear Instability, reconciling theoretical models with observations of highly settled outer disk regions.

Contribution

It demonstrates how dust coagulation reduces gas cooling rates, extinguishing VSI activity and aligning simulations with observed disk structures, highlighting the role of fragmentation velocity.

Findings

Dust coagulation diminishes gas cooling, suppressing VSI turbulence.

Lower fragmentation velocities (~100 cm/s) lead to turbulent disks.

Higher fragmentation velocities (~400 cm/s) result in laminar, settled disks.

Abstract

Protoplanetary disks exhibit a vertical gradient in angular momentum, rendering them susceptible to the Vertical Shear Instability (VSI). The most important condition for the onset of this mechanism is a short timescale of thermal relaxation ($\lesssim 0.1$ orbital timescales). Simulations of fully VSI active disks are characterized by turbulent, vertically extended dust layers. This is in contradiction with recent observations of the outer regions of some protoplanetary disks, which appear highly settled. In this work, we demonstrate that the process of dust coagulation can diminish the cooling rate of the gas in the outer disk and extinct the VSI activity. Our findings indicate that the turbulence strength is especially susceptible to variations in the fragmentation velocity of the grains. A small fragmentation velocity of $\approx 100 \mathrm{\, cm \,s^{-1}}$ results in a fully turbulent simulation, whereas a value of $\approx 400 \mathrm{\, cm \,s^{-1}}$ results in a laminar outer disk, being consistent with observations. We show that VSI turbulence remains relatively unaffected by variations in the maximum particle size in the inner disk regions. However, we find that dust coagulation can significantly suppress the occurrence of VSI turbulence at larger distances from the central star.