An Ordered Magnetic Field in the Protoplanetary Disk of AB Aur Revealed by Mid-Infrared Polarimetry

TL;DR

This study uses mid-infrared polarimetry to reveal an ordered magnetic field in the protoplanetary disk of AB Aur, providing observational evidence of magnetic influence on disk surface layers.

Contribution

First direct detection of an ordered magnetic field in the surface layer of a protoplanetary disk via mid-infrared polarimetry.

Findings

Magnetic field is well ordered and has a poloidal shape tilted from the disk axis.

Inner disk polarization arises from dichroic emission of aligned dust grains.

Beyond 80 AU, polarization is dominated by dust scattering, indicating grain growth.

Abstract

Magnetic fields (B-fields) play a key role in the formation and evolution of protoplanetary disks, but their properties are poorly understood due to the lack of observational constraints. Using CanariCam at the 10.4-m Gran Telescopio Canarias, we have mapped out the mid-infrared polarization of the protoplanetary disk around the Herbig Ae star AB Aur. We detect ~0.44% polarization at 10.3 micron from AB Aur's inner disk (r < 80 AU), rising to ~1.4% at larger radii. Our simulations imply that the mid-infrared polarization of the inner disk arises from dichroic emission of elongated particles aligned in a disk B-field. The field is well ordered on a spatial scale commensurate with our resolution (~50 AU), and we infer a poloidal shape tilted from the rotational axis of the disk. The disk of AB Aur is optically thick at 10.3 micron, so polarimetry at this wavelength is probing the B-field near the disk surface. Our observations therefore confirm that this layer, favored by some theoretical studies for developing magneto-rotational instability and its resultant viscosity, is indeed very likely to be magnetized. At radii beyond ~80 AU, the mid-infrared polarization results primarily from scattering by dust grains with sizes up to ~1 micron, a size indicating both grain growth and, probably, turbulent lofting of the particles from the disk mid-plane.