Magnetically Induced Disk Winds and Transport in the HL Tau Disk

TL;DR

This paper investigates how magnetically induced disk winds can explain the observed dust settling and high accretion rates in the HL Tau protoplanetary disk, challenging traditional viscous models.

Contribution

It introduces a semi-analytical model incorporating non-ideal MHD simulations to explain HL Tau's disk properties with magnetic disk winds.

Findings

Disk winds can account for HL Tau's high accretion and dust settling.

Steady accretion requires a plasma beta of about 2×10^4 at the midplane.

Vertical magnetic flux at 100 au is approximately 0.2 mG.

Abstract

The mechanism of angular momentum transport in protoplanetary disks is fundamental to understand the distributions of gas and dust in the disks. The unprecedented, high spatial resolution ALMA observations taken toward HL Tau and subsequent radiative transfer modeling reveal that a high degree of dust settling is currently achieved at the outer part of the HL Tau disk. Previous observations however suggest a high disk accretion rate onto the central star. This configuration is not necessarily intuitive in the framework of the conventional viscous disk model, since efficient accretion generally requires a high level of turbulence, which can suppress dust settling considerably. We develop a simplified, semi-analytical disk model to examine under what condition these two properties can be realized in a single model. Recent, non-ideal MHD simulations are utilized to realistically model the angular momentum transport both radially via MHD turbulence and vertically via magnetically induced disk winds. We find that the HL Tau disk configuration can be reproduced well when disk winds are properly taken into account. While the resulting disk properties are likely consistent with other observational results, such an ideal situation can be established only if the plasma $\beta$ at the disk midplane is $\beta_0 \simeq 2 \times 10^4$ under the assumption of steady accretion. Equivalently, the vertical magnetic flux at 100 au is about 0.2 mG. More detailed modeling is needed to fully identify the origin of the disk accretion and quantitatively examine plausible mechanisms behind the observed gap structures in the HL Tau disk.