2019ApJ...883...16M

We present an 870 µm Atacama Large Millimeter Array polarization observation toward the Class II protoplanetary disk around AS 209, which has concentric, multiple gaps and rings. We successfully detect the polarized emission and find that the polarization orientations and fractions have distinct characteristics between the inner and outer regions. In the inner region, the polarization orientations are parallel to the minor axis of the disk, which is consistent with the self-scattering model. The mean polarization fraction in the region is ∼0.2%, which is lower than the expected value when the grains have the maximum polarization efficiency, which corresponds to λ/2π ∼ 140 µm in grain radius. In the outer region, we detect ∼1.0% polarization and find that the polarization orientations are almost in the azimuthal directions. Moreover, the polarization orientations have systematic angular deviations from the azimuthal directions with Δθ ∼ 4.°5 ± 1.°6. The pattern is consistent with a model in which radially drifting dust grains are aligned by the gas flow against the dust grains. We consider possible scenarios of the grain dynamics at the AS 209 ring that can reproduce the polarization pattern. However, the directions of the observed angular deviations are opposite to what is predicted based on the fact that the disk rotates clockwise. This raises a question regarding our understanding of the alignment processes and/or grain dynamics in protoplanetary disks.