What Produces Dust Polarization in the HH 212 Protostellar Disk at 878
  {\mu}m: Dust Self-Scattering or Dichroic Extinction?
  We report new dust polarization results of a nearly edge-on disk in the HH
212 protostellar system, obtained with ALMA at ~ 0.035" (14 au) resolution in
continuum at lambda ~ 878 um. Dust polarization is detected within ~ 44 au of
the central source, where a rotationally supported disk has formed. The
polarized emission forms V-shaped structures opening to the east and probably
west arising from the disk surfaces and arm structures further away in the east
and west that could be due to potential spiral arms excited in the outer disk.
The polarization orientations are mainly parallel to the minor axis of the
disk, with some in the western part tilting slightly away from the minor axis
to form a concave shape with respect to the center. This tilt of polarization
orientations is expected from dust self-scattering, e.g., by 50-75 um grains in
a young disk. The polarized intensity and polarization degree both peak near
the central source with a small dip at the central source and decrease towards
the edges. These decreases of polarized intensity and polarization degree are
expected from dichroic extinction by grains aligned by poloidal fields, but may
also be consistent with dust self-scattering if the grain size decreases toward
the edges. It is possible that both mechanisms are needed to produce the
observed dust polarization, suggesting the presence of both grain growth and
poloidal fields in the disk.
