Unveiling the importance of magnetic fields in the evolution of dense
  clumps formed at the waist of bipolar H II regions: a case study on Sh2-201
  with JCMT SCUBA-2/POL-2
  We present the properties of magnetic fields (B-fields) in two clumps (clump
1 and clump 2), located at the waist of the bipolar H II region Sh2-201, based
on JCMT SCUBA-2/POL-2 observations of 850 $\mu$m polarized dust emission. We
find that B-fields in the direction of the clumps are bent and compressed,
showing bow-like morphologies, which we attribute to the feedback effect of the
H II region on the surface of the clumps. Using the modified
Davis-Chandrasekhar-Fermi method we estimate B-fields strengths of 266 $\mu$G
and 65 $\mu$G for clump 1 and clump 2, respectively. From virial analyses and
critical mass ratio estimates, we argue that clump 1 is gravitationally bound
and could be undergoing collapse, whereas clump 2 is unbound and stable. We
hypothesize that the interplay between thermal pressure imparted by the H II
region, B-field morphologies, and the various internal pressures of the clumps
(such as magnetic, turbulent, and gas thermal pressure), has the following
consequences: (a) formation of clumps at the waist of the H II region; (b)
progressive compression and enhancement of the B-fields in the clumps; (c)
stronger B-fields will shield the clumps from erosion by the H II region and
cause pressure equilibrium between the clumps and the H II region, thereby
allowing expanding I-fronts to blow away from the filament ridge, forming
bipolar H II regions; and (d) stronger B-fields and turbulence will be able to
stabilize the clumps. A study of a larger sample of bipolar H II regions would
help to determine whether our hypotheses are widely applicable.
