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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

Abstract : 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 \ensuremathμm 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 \ensuremathμG 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.
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Submitted on : Tuesday, February 8, 2022 - 5:44:43 PM
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Chakali Eswaraiah, Di Li, Manash R. Samal, Jia-Wei Wang, Yuehui Ma, et al.. 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. The Astrophysical Journal, American Astronomical Society, 2020, 897 (1), pp.90. ⟨10.3847/1538-4357/ab83f2⟩. ⟨hal-03561804⟩

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