Oral Presentation

Probing Magnetic Fields in Protoplanetary Disk Atmospheres through Polarized near-IR Light Scattered by Aligned Grains

Author(s): Haifeng Yang

Presenter: Haifeng Yang (Kavli Institute for Astronomy and Astrophysics, Peking University)

The magnetic field plays an essential role in the evolution and dynamics of protoplanetary disks.
The magnetic field in the disk atmosphere is of particular interest, as it is directly connected to the wind-launching mechanism.
In this work, we study the polarization of the light scattered off of magnetically aligned grains in the disk atmosphere, focusing on the deviation of the polarization orientation from the canonical azimuthal direction, which may be detectable in near-IR with instruments such as VLT/SPHERE.
We first study the polarization in the grain's frame (GF), finding that the angle difference, the deviation from azimuthal orientation, increases with an increasing inclination angle (in GF) and dust aspect ratio, and can easily reach 10 degrees.
We show with a simple disk model that the polarization can even be oriented along the radial (rather than azimuthal) direction, especially in a highly inclined disk with a toroidally dominated magnetic field. This polarization reversal is caused by the anisotropy in the polarizibility of aligned grains and is thus a telltale sign of such grains.
We show that the near-IR light is scattered mostly by grains of micron sizes or smaller at the tau=1 surface and such grains can be magnetically aligned if they contain superparamagnetic inclusions. For comparison with polarimetric observations, we generate synthetic maps of the ratios of Stokes parameters U_phi/I and Q_phi/I, which can be used to infer the existence of (magnetically) aligned grains through a negative Q_\phi (polarization reversal) and/or a significant level of U_\phi/I.
Our mechanism is easy to tell from other U_phi-producing mechanisms, such as the multiple scattering, because the peak location of U_phi is different. Our mechanism also depends on the helicity of magnetic fields.