Spatially Resolved Magnetic Field Structure in the Disk of a T Tauri Star

TL;DR

This study presents high-resolution observations of magnetic fields in the disk of a T Tauri star, revealing complex field structures that challenge existing models of star formation and disk accretion.

Contribution

First resolved measurement of magnetic field morphology in a T Tauri star's disk, providing new insights into magnetic field configurations during star formation.

Findings

Magnetic field aligned with the disk's major axis.

Purely toroidal or vertical fields do not fit the data.

Magnetic field morphology is more complex than current models suggest.

Abstract

Magnetic fields in accretion disks play a dominant role during the star formation process but have hitherto been observationally poorly constrained. Field strengths have been inferred on T Tauri stars themselves and possibly in the innermost part of the accretion disk, but the strength and morphology of the field in the bulk of the disk have not been observed. Unresolved measurements of polarized emission (arising from elongated dust grains aligned perpendicular to the field) imply average fields aligned with the disks. Theoretically, the fields are expected to be largely toroidal, poloidal, or a mixture of the two, which imply different mechanisms for transporting angular momentum in the disks of actively accreting young stars such as HL Tau. Here we report resolved measurements of the polarized 1.25 mm continuum emission from HL Tau's disk. The magnetic field on a scale of 80 AU is coincident with the major axis (~210 AU diameter) of the disk. From this we conclude that the magnetic field inside the disk at this scale cannot be dominated by a vertical component, though a purely toroidal field does not fit the data well either. The unexpected morphology suggests that the magnetic field's role for the accretion of a T Tauri star is more complex than the current theoretical understanding.