The evidence of radio polarization induced by the radiative grain
  alignment and self-scattering of dust grains in a protoplanetary disk
  The mechanisms causing millimeter-wave polarization in protoplanetary disks
are under debate. To disentangle the polarization mechanisms, we observe the
protoplanetary disk around HL Tau at 3.1 mm with the Atacama Large
Millimeter/submillimeter Array (ALMA), which had polarization detected with
CARMA at 1.3 mm. We successfully detect the ring-like azimuthal polarized
emission at 3.1 mm. This indicates that dust grains are aligned with the major
axis being in the azimuthal direction, which is consistent with the theory of
radiative alignment of elongated dust grains, where the major axis of dust
grains is perpendicular to the radiation flux. Furthermore, the morphology of
the polarization vectors at 3.1 mm is completely different from those at 1.3
mm. We interpret that the polarization at 3.1 mm to be dominated by the grain
alignment with the radiative flux producing azimuthal polarization vectors,
while the self-scattering dominates at 1.3 mm and produces the polarization
vectors parallel to the minor axis of the disk. By modeling the total
polarization fraction with a single grain population model, the maximum grain
size is constrained to be $100{\rm~\mu m}$, which is smaller than the previous
predictions based on the spectral index between ALMA at 3 mm and VLA at 7 mm.
