Accretion bursts in magnetized gas-dust protoplanetary disks

TL;DR

This study uses magnetohydrodynamics simulations to investigate how magnetorotational instability triggers accretion bursts in the inner regions of protoplanetary disks, considering dust growth, magnetic fields, and ionization effects over long timescales.

Contribution

It introduces a comprehensive model of protoplanetary disk evolution that includes dust-gas interactions, magnetic effects, and MRI-triggered accretion bursts with variable ionization and turbulence.

Findings

Dead zones form with low ionization and temperature, leading to dust ring formation.

MRI-triggered bursts occur due to thermal ionization in dead zones, causing rapid accretion events.

Burst frequency is highest early on and linked to gravitational instability driven by mass infall.

Abstract

Aims and Methods. Accretion bursts triggered by the magnetorotational instability (MRI) in the innermost disk regions were studied for protoplanetary gas-dust disks formed from prestellar cores of various mass $M_{\rm core}$ and mass-to-magnetic flux ratio $\lambda$. Numerical magnetohydrodynamics simulations in the thin-disk limit were employed to study the long-term ($\sim 1.0$~Myr) evolution of protoplanetary disks with an adaptive turbulent $\alpha$-parameter, which depends explicitly on the strength of the magnetic field and ionization fraction in the disk. The numerical models also feature the co-evolution of gas and dust, including the back-reaction of dust on gas and dust growth. Results. Dead zone with a low ionization fraction $x <= 10^{-13}$ and temperature on the order of several hundred Kelvin forms in the inner disk soon after its formation, extending from several to several tens of astronomical units depending on the model. The dead zone features pronounced dust rings that are formed due to the concentration of grown dust particles in the local pressure maxima. Thermal ionization of alkaline metals in the dead zone trigger the MRI and associated accretion burst, which is characterized by a sharp rise, small-scale variability in the active phase, and fast decline once the inner MRI-active region is depleted of matter. The burst occurrence frequency is highest in the initial stages of disk formation, and is driven by gravitational instability (GI), but declines with diminishing disk mass-loading from the infalling envelope. There is a causal link between the initial burst activity and the strength of GI in the disk fueled by mass infall from the envelope. Abridged.