Expected Polarization at Sub-mm Wavelengths from Lopsided Protoplanetary Disks

There has been few significant detections of linear polarization of sub-mm continuum from protoplanetary disks. Polarization from star-forming cores has been interpreted as thermal emission from dust grains aligned with magnetic fields. Thus, the mechanism of polarization from protoplanetary disks has also been expected to be the aligned grains. In this work, however, we show that dust scattering accounts for the sub-mm polarization of protoplanetary disks. We use 3D-radiative transfer calculations to evaluate the polarization of a lopsided protoplanetary disk. As a result, we find that the polarization by dust scattering is detectable by ALMA Band 7 polarization observations. If the maximum dust grain size is 100 mu m, the polarization by dust scattering is 2.5 % at the peak of the polarized intensity. If the polarization is detected, it will be a direct evidence of grain growth.