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arXiv · 1912.10188

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

Chakali Eswaraiah·Di Li·Manash R. Samal·Jia-Wei Wang·Yuehui Ma·Shih-Ping Lai·Annie Zavagno·Tao-Chung Ching·Tie Liu·Kate Pattle·Derek Ward-Thompson·Anil K. Pandey·Devendra K. Ojha
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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 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.

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Chakali Eswaraiah, Di Li, Manash R. Samal, Jia-Wei Wang, Yuehui Ma, Shih-Ping Lai, Annie Zavagno, Tao-Chung Ching, Tie Liu, Kate Pattle, Derek Ward-Thompson, Anil K. Pandey, Devendra K. Ojha. 2019-12-21. 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. https://doi.org/10.3847/1538-4357%2Fab83f2

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