Polarization Properties and Magnetic Field Structures in the High-Mass
  Star-Forming Region W51 Observed with ALMA
  We present the first ALMA dust polarization observations towards the
high-mass star-forming regions W51 e2, e8, and W51 North in Band 6 (230 GHz)
with a resolution around 0.26" ($\sim5$mpc). Polarized emission in all three
sources is clearly detected and resolved. Measured relative polarization levels
are between 0.1\% and 10\%. While the absolute polarization shows complicated
structures, the relative polarization displays the typical anti-correlation
with Stokes $I$, though with a large scatter. Inferred magnetic (B) field
morphologies are organized and connected. Detailed substructures are resolved,
revealing new features such as cometary-shaped B-field morphologies in
satellite cores, symmetrically converging B-field zones, and possibly
streamlined morphologies. The local B-field dispersion shows some
anti-correlation with the relative polarization. Moreover, lowest polarization
percentages together with largest dispersions coincide with B-field convergence
zones. We put forward $\sin\omega$, where $\omega$ is the measurable angle
between a local B-field orientation and local gravity, as a measure of how
effectively the B-field can oppose gravity. Maps of $\sin\omega$ for all three
sources show organized structures that suggest a locally varying role of the
B-field, with some regions where gravity can largely act unaffectedly, possibly
in a network of narrow magnetic channels, and other regions where the B-field
can work maximally against gravity.
