Dust Polarization Toward Embedded Protostars in Ophiuchus with ALMA. I.
  VLA 1623
  We present high resolution (~ 30 au) ALMA Band 6 dust polarization
observations of VLA 1623. The VLA 1623 data resolve compact ~ 40 au inner disks
around the two protobinary sources, VLA 1623-A and VLA 1623-B, and also an
extended ~ 180 au ring of dust around VLA 1623-A. This dust ring was previously
identified as a large disk in lower-resolution observations. We detect highly
structured dust polarization toward the inner disks and the extended ring with
typical polarization fractions ~ 1.7% and ~ 2.4%, respectively. The two
components also show distinct polarization morphologies. The inner disks have
uniform polarization angles aligned with their minor axes. This morphology is
consistent with expectations from dust scattering. By contrast, the extended
dust ring has an azimuthal polarization morphology not previously seen in
lower-resolution observations. We find that our observations are well fit by a
static, oblate spheroid model with a flux-frozen, poloidal magnetic field. We
propose that the polarization traces magnetic grain alignment likely from flux
freezing on large scales and magnetic diffusion on small scales. Alternatively,
the azimuthal polarization may be attributed to grain alignment by the
anisotropic radiation field. If the grains are radiatively aligned, then our
observations indicate that large (~ 100 um) dust grains grow quickly at large
angular extents. Finally, we identify significant proper motion of VLA 1623
using our observations and those in the literature. This result indicates that
the proper motion of nearby systems must be corrected for when combining ALMA
data from different epochs.
