Unveiling the Importance of Magnetic Fields in the Evolution of Dense Clumps Formed at the Waist of Bipolar H ii Regions: A Case Study of Sh 2-201 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 Sh 2-201, based on James Clerk Maxwell Telescope SCUBA-2/POL-2 observations of 850 μm 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-field strengths of 266 and 65 μG 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 of the thermal pressure imparted by the H ii region, the B-field morphologies, and the various internal pressures of the clumps (such as magnetic, turbulent, and gas thermal pressures) 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 that 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 ionization fronts to blow away from the filament ridge, forming bipolar H ii regions; and (d) stronger B fields and turbulence that 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.