Vertical shear instability with dust evolution and consistent cooling times. On the importance of the initial dust distribution

TL;DR

This study investigates how initial dust grain sizes influence the vertical shear instability (VSI) and thermal relaxation in protoplanetary disks, revealing that larger grains can suppress turbulence in outer regions and that VSI can be sustained over long timescales.

Contribution

It provides the first dynamic modeling of the interplay between dust evolution, thermal relaxation, and VSI in protoplanetary disks, highlighting the importance of initial dust size distributions.

Findings

Larger initial dust grains suppress VSI in outer disk regions.

VSI can be sustained over long timescales despite ongoing dust growth.

Dust distribution and grain size critically influence turbulence development.

Abstract

Context. Gas in protoplanetary disks mostly cools via thermal accommodation with dust particles. Thermal relaxation is thus highly sensitive to the local dust size distributions and the spatial distribution of the grains. So far, the interplay between thermal relaxation and gas turbulence has not been dynamically modeled in hydrodynamic simulations of protoplanetary disks with dust. Aims. We aim to study the effects of the vertical shear instability (VSI) on the thermal relaxation times, and vice versa. We are particularly interested in the influence of the initial dust grain size on the VSI and whether the emerging turbulence is sustained over long timescales. Results. We find that the emergence of the VSI is strongly dependent on the initial dust grain size. Coagulation also counteracts the emergence of hydrodynamic turbulence in our simulations, as shown by others before. Starting a simulation with larger grains (100 $\mu$m) generally leads to a less turbulent outcome. While the inner disk regions (within $\sim$ 70 au) develop turbulence in all three simulations, we find that the simulations with larger particles do not develop VSI in the outer disk. Conclusions. Our simulations with dynamically calculated thermal accommodation times based on the drifting and settling dust distribution show that the VSI, once developed in a disk, can be sustained over long timescales, even if grain growth is occurring. The VSI corrugates the dust layer and even diffuses the smaller grains into the upper atmosphere, where they can cool the gas. Whether the instability can emerge for a specific stratification depends on the initial dust grain sizes and the initial dust scale height. If the grains are initially $\gtrsim$ 100 $\mu$m and if the level of turbulence is initially assumed to be low, we find no VSI turbulence in the outer disk regions.