An Ordered Magnetic Field in the Protoplanetary Disk of AB Aur Revealed by Mid-infrared Polarimetry
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 μm 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 μm, 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 μm, a size indicating both grain growth and, probably, turbulent lofting of the particles from the disk mid-plane.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2016
- DOI:
- 10.3847/0004-637X/832/1/18
- arXiv:
- arXiv:1609.02493
- Bibcode:
- 2016ApJ...832...18L
- Keywords:
-
- magnetic fields;
- polarization;
- protoplanetary disks;
- stars: individual: AB Aur;
- stars: pre-main sequence;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 10 pages, 5 figures, accepted for publication in ApJ