Global 3-D Radiation Magnetohydrodynamic Simulations for FU Ori's Accretion Disk and Observational Signatures of Magnetic Fields

TL;DR

This study uses global 3D radiation MHD simulations to explore magnetic fields and wind launching mechanisms in FU Ori's accretion disk, aligning with observations and revealing new insights into disk structure and magnetic field configurations.

Contribution

It provides the first comprehensive global 3D radiation MHD simulations of FU Ori's inner disk, demonstrating the impact of magnetic fields on accretion and wind launching, and comparing results with observational data.

Findings

Magnetically dominated atmosphere facilitates surface accretion.

Moderate disk wind with observed-like properties is launched in vertical magnetic fields.

Magnetic field structures vary between wind launching and accreting regions.

Abstract

FU Ori is the prototype of FU Orionis systems which are outbursting protoplanetary disks. Magnetic fields in FU Ori's accretion disks have previously been detected using spectropolarimetry observations for Zeeman effects. We carry out global radiation ideal MHD simulations to study FU Ori's inner accretion disk. We find that (1) when the disk is threaded by vertical magnetic fields, most accretion occurs in the magnetically dominated atmosphere at z$\sim$R, similar to the "surface accretion" mechanism in previous locally-isothermal MHD simulations. (2) A moderate disk wind is launched in the vertical field simulations with a terminal speed of $\sim$300-500 km/s and a mass loss rate of 1-10\% the disk accretion rate, which is consistent with observations. Disk wind fails to be launched in simulations with net toroidal magnetic fields. (3) The disk photosphere at the unit optical depth can be either in the wind launching region or the accreting surface region. Magnetic fields have drastically different directions and magnitudes between these two regions. Our fiducial model agrees with previous optical Zeeman observations regarding both the field directions and magnitudes. On the other hand, simulations indicate that future Zeeman observations at near-IR wavelengths or towards other FU Orionis systems may reveal very different magnetic field structures. (4) Due to energy loss by the disk wind, the disk photosphere temperature is lower than that predicted by the thin disk theory, and the previously inferred disk accretion rate may be lower than the real accretion rate by a factor of $\sim$2-3.