Fengwei Xu, Q. Zhang, P. Sanhueza, K. Wang, Hauyu Baobab Liu, H. Beuther, Wenyu Jiao, C. Wang, P. C. Cortés, P. M. Koch, J. M. Girart, M. T. Beltrán, J.-W. Wang, J. Liu, F. A. Olguin, Xing Lü, S. Li, Pak Shing Li, T. Liu, K. Morii, J. Hwang, H.-R. V. Chen, S. Jiao, Y. Cheng, Q. Luo, Piyali Saha, Ji‐hyun Kang, C. Y. Law, L. K. Dewangan, O. R. Jadhav, E. J. Chung, Chakali Eswaraiah, Luis A. Zapata
Our understanding of magnetic fields (B-fields) in massive star formation remains incomplete. Linear polarized emission from magnetically aligned dust grains provides a good way to map the morphology of the B-field on the plane of the sky. Here, we present the 1.2 mm full polarization observation of W33 A, a massive star-forming region at 2.4 kpc, obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA) to achieve an angular resolution of ∼0 (∼730 au). W33 A is resolved into 20 dense cores and nine filaments. It reveals various B-field structures, including two perpendicular large-scale components oriented northwest-southeast (NW-SE) and northeast--southwest (NE-SW), as well as two local distinct features toward the millimeter peaks MM1 and MM2. The NW-SE component could be shaped by a molecular outflow. The NE-SW one is remarkably coherent along the main filamentary structures (F1, F-Main, Tail), all with trans-Alfvénic turbulence. In F-Main, the line mass exceeds the turbulent critical limits, so magnetic support is required to prevent radial collapse and suppress local fragmentation. In F1 and Tail, turbulence is sufficient to support gravity, although B-fields can potentially provide additional support. Toward MM1, the fields follow a spiral-like, infalling streamer traced by CH_3CN; the inferred trans-Alfvénic turbulence in the accreting material indicates magnetic damping of turbulence and a magnetically regulated, laminar accretion flow that continues to feed the core. Toward MM2, the field exhibits an hourglass geometry described by parabolic curves. Two independent methods yield a consistently strong field strength of ∼!8.1 mG. The virial analysis shows that the B-field can add 25% (75% from turbulence) support against gravity but is not by itself sufficient to halt collapse. Our study shows that within one protocluster, B-fields can both help stabilize gas filament against local fragmentation to facilitate mass accretion and delay gravitational collapse. The distinct evolutionary stages of MM1 and MM2 highlight the dynamic importance of B-fields in massive star formation.