# The impact of dust evolution on the dead zone outer edge in magnetized protoplanetary disks

**Authors:** Timmy N. Delage, Matías Gárate, Satoshi Okuzumi, Chao-Chin Yang, Paola Pinilla, Mario Flock, Sebastian Markus Stammler, Tilman Birnstiel  
**arXiv:** [2303.15675](https://arxiv.org/abs/2303.15675)  
**Date:** 2023-03-28  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.15675v1/

## Key points

- The study aims to understand the magnetorotational instability (MRI)-dust coevolution in protoplanetary disks.
- Dust evolution plays a crucial role in MRI activity.
- Impact of fixed power-law dust size distribution on MRI activity using an improved turbulence model
- Coupling the turbulence model with a dust evolution code called DustPy to study how dust evolution and MRI-driven accretion are intertwined on million-year timescales.
- Dust coagulation and settling lead to stronger MRI-driven turbulence and a more compact dead zone, while fragmentation has the opposite effect.
- As the dust content decreases over millions of years due to radial drift, overall MRI-driven turbulence becomes stronger until behaving as a grain-free plasma. However, dust evolution alone does not lead to complete reactivation of the dead zone.
- Temporal evolution of MRI-induced alpha-parameter is controlled by dust evolution on timescales of local dust growth as long as there are enough dust particles in the disk. Gas evolution takes over control on viscous evolution timescales.
- Changes in magnetic field strength and configuration or large-scale instabilities can temporarily suppress or enhance MRI activity.
- The study highlights the need for further investigation into these processes.

## Summary

This study aims to provide a better understanding of the magnetorotational instability (MRI)-dust coevolution in protoplanetary disks and presents a proof of concept that dust evolution plays a crucial role in the MRI activity. The analysis is divided into two parts: First, the impact of a fixed power-law dust size distribution with varying parameters on the MRI activity, especially steady-state MRI-driven accretion, is studied using an improved 1+1D MRI-driven turbulence model. Second, the steady-state accretion assumption is relaxed in this newly improved turbulence model and partially coupled to a dust evolution code called DustPy. This allows for insights into how dust evolution and MRI-driven accretion are intertwined on million-year timescales. The results show that dust coagulation and settling lead to a higher gas ionization degree, resulting in stronger MRI-driven turbulence as well as a more compact dead zone. On the other hand, fragmentation has an opposite effect because it replenishes the disk with small dust particles that sweep up free electrons and ions from the gas phase. As the dust content of the protoplanetary disk decreases over millions of years due to radial drift, overall MRI-driven turbulence becomes stronger and the dead zone more compact until eventually behaving as a grain-free plasma. However, these results also indicate that dust evolution alone does not lead to complete reactivation of the dead zone. The study concludes that the temporal evolution of MRI-induced $\alpha$-parameter is controlled by dust evolution on timescales of local dust growth as long as there are enough dust particles in the disk to dominate recombination processes for ionization chemistry. Once this is no longer true, gas evolution takes over control on viscous evolution timescales. Additionally, it was found that another common feature between all models was that when magnetic field strength and configuration change significantly compared to initial conditions or when large-scale instabilities occur (e.g., gravitational instability), the MRI activity can be temporarily suppressed or enhanced. Overall, this study provides important insights into the complex interplay between dust evolution and MRI-driven accretion in protoplanetary disks and highlights the need for further investigation into these processes.