ALMA observations of polarized emission toward the CW Tau and DG Tau
  protoplanetary disks: constraints on dust grain growth and settling
  We present polarimetric data of CW Tau and DG Tau, two well-known Class II
disk/jet systems, obtained with the Atacama Large Millimeter/submillimeter
Array at 870 $\mu$m and 0."2 average resolution. In CW Tau, the total and
polarized emission are both smooth and symmetric, with polarization angles
almost parallel to the minor axis of the projected disk. In contrast, DG Tau
displays a structured polarized emission, with an elongated brighter region in
the disk's near side and a belt-like feature beyond about 0."3 from the source.
At the same time the total intensity is spatially smooth, with no features. The
polarization pattern, almost parallel to the minor axis in the inner region,
becomes azimuthal in the outer belt, possibly because of a drop in optical
depth. The polarization fraction has average values of 1.2% in CW Tau and 0.4%
in DG Tau. Our results are consistent with polarization from self-scattering of
the dust thermal emission. Under this hypothesis, the maximum size of the
grains contributing to polarization is in the range 100 - 150 $\mu$m for CW Tau
and 50 - 70 $\mu$m for DG Tau. The polarization maps combined with dust opacity
estimates indicate that these grains are distributed in a geometrically thin
layer in CW Tau, representing a settling in the disk midplane. Meanwhile, such
settling is not yet apparent for DG Tau. These results advocate polarization
studies as a fundamental complement to total emission observations, in
investigations of the structure and the evolution of protoplanetary disks.
