Multi-scale Dust Polarization and Spiral-like Stokes-I Residual in the
  Class I Protostellar System TMC-1A
  We have observed the Class I protostar TMC-1A in the Taurus molecular cloud
using the Submillimeter Array (SMA) and the Atacama Large
Millimeter/submillimeter Array (ALMA) in the linearly polarized 1.3 mm
continuum emission at angular resolutions of ~3" and ~0.3", respectively. The
ALMA observations also include CO, 13CO, and C18O J=2-1 spectral lines. The SMA
observations trace magnetic fields on the 1000-au scale, the directions of
which are neither parallel nor perpendicular to the outflow direction. Applying
the Davis-Chandrasekhar-Fermi method to the SMA polarization angle dispersion,
we estimate a field strength in the TMC-1A envelope of 1-5 mG. It is consistent
with the field strength needed to reduce the radial infall velocity to the
observed value, which is substantially less than the local} free-fall velocity.
The ALMA polarization observations consist of two distinct components -- a
central component and a north/south component. The central component shows
polarization directions in the disk minor axis to be azimuthal, suggesting dust
self-scattering in the TMC-1A disk. The north/south component is located along
the outflow axis and the polarization directions are aligned with the outflow
direction. We discuss possible origins of this polarization structure,
including grain alignment by a toroidal magnetic field and mechanical alignment
by the gaseous outflow. In addition, we discover a spiral-like residual in the
total intensity (Stokes I) for the first time. The C18O emission suggests that
material in the spiral-like structure is infalling at a speed that is 20% of
the local Keplerian speed.
