ALMA Reveals Transition of Polarization Pattern with Wavelength in HL
  Tau's Disk
  The mechanism for producing polarized emission from protostellar disks at
(sub)millimeter wavelengths is currently uncertain. Classically, polarization
is expected from non-spherical grains aligned with the magnetic field.
Recently, two alternatives have been suggested. One polarization mechanism is
caused by self-scattering from dust grains of sizes comparable to the
wavelength while the other mechanism is due to grains aligned with their short
axes along the direction of radiation anisotropy. The latter has recently been
shown as a likely mechanism for causing the dust polarization detected in HL
Tau at 3.1 mm. In this paper, we present ALMA polarization observations of HL
Tau for two more wavelengths: 870 $\mu$m and 1.3 mm. The morphology at 870
$\mu$m matches the expectation for self-scattering, while that at 1.3 mm shows
a mix between self-scattering and grains aligned with the radiation anisotropy.
The observations cast doubt on the ability of (sub)millimeter continuum
polarization to probe disk magnetic fields for at least HL Tau. By showing two
distinct polarization morphologies at 870 $\mu$m and 3.1 mm and a transition
between the two at 1.3 mm, this paper provides definitive evidence that the
dominant (sub)millimeter polarization mechanism transitions with wavelength. In
addition, if the polarization at 870 $\mu$m is due to scattering, the lack of
polarization asymmetry along the minor axis of the inclined disk implies that
the large grains responsible for the scattering have already settled into a
geometrically thin layer, and the presence of asymmetry along the major axis
indicates that the HL Tau disk is not completely axisymmetric.
