http://arxiv.org/abs/2203.03213
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 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 optical/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 $\sim 10^\circ$. 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 anistropy in the polarizibility of aligned grains and is thus a telltale sign of such grains. We show that the optical/IR light is scattered mostly by grains of $\mu$m 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 $U_\phi/I$.
H. Yang and Z. Li
Tue, 8 Mar 22
22/100
Comments: 18 pages, 21 figures, submitted to AAS Journals. Comments are welcome