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

Chakali Eswaraiah (1) , Di Li (2) , Manash R. Samal (3) , Jia-Wei Wang (4) , Yuehui Ma (5) , Shih-Ping Lai (4) , Annie Zavagno (6) , Tao-Chung Ching (1) , Tie Liu (7) , Kate Pattle (4, 8) , Derek Ward-Thompson (9) , Anil K. Pandey (10) , Devendra K. Ojha (11)

1 NAOC - National Astronomical Observatories [Beijing]
2 PERSEE - Centre Procédés, Énergies Renouvelables, Systèmes Énergétiques
3 PRL - Physical Research Laboratory [Ahmedabad]
4 NTHU - National Tsing Hua University [Hsinchu]
5 ASIAA - Academia Sinica Institute of Astronomy and Astrophysics
6 LAM - Laboratoire d'Astrophysique de Marseille
7 XIEG - Xinjiang Institute of Ecology and Geography [Urumqi]
8 NUI Galway - National University of Ireland [Galway]
9 Jeremiah Horrocks Institute for Mathematics, Physics and Astronomy [Preston]
10 ARIES - Aryabhatta Research Institute of Observational Sciences
11 TIFR - Tata Institute for Fundamental Research

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.

Domaines: Planète et Univers [physics]

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, 2020, 897 (1), pp.90. ⟨10.3847/1538-4357/ab83f2⟩. ⟨hal-03561804⟩