Understanding the links among magnetic fields, filament, the bipolar bubble, and star formation in RCW57A using NIR polarimetry

TL;DR

This study uses near-infrared polarimetry to explore how magnetic fields influence the formation of bipolar bubbles, filaments, and star formation in the RCW57A region, revealing magnetic fields' dominant role in shaping structures.

Contribution

It provides the first detailed magnetic field mapping in RCW57A, demonstrating their influence on filament formation, bubble expansion, and star formation processes.

Findings

Magnetic fields are perpendicular to the filament and follow the bipolar bubble structure.

Mean magnetic field strength is estimated at 91 μG, dominating over turbulence and thermal pressures.

Magnetic pressure influences the expansion of ionization fronts and star formation in RCW57A.

Abstract

The influence of magnetic fields (B-fields) in the formation and evolution of bipolar bubbles, due to the expanding ionization fronts (I-fronts) driven by the Hii regions that are formed and embedded in filamentary molecular clouds, has not been well-studied yet. In addition to the anisotropic expansion of I-fronts into a filament, B-fields are expected to introduce an additional anisotropic pressure which might favor expansion and propagation of I-fronts to form a bipolar bubble. We present results based on near-infrared polarimetric observations towards the central $\sim$8'$\times$8' area of the star-forming region RCW57A which hosts an Hii region, a filament, and a bipolar bubble. Polarization measurements of 178 reddened background stars, out of the 919 detected sources in the JHKs-bands, reveal B-fields that thread perpendicular to the filament long axis. The B-fields exhibit an hour-glass morphology that closely follows the structure of the bipolar bubble. The mean B-field strength, estimated using the Chandrasekhar-Fermi method, is 91$\pm$8 {\mu}G. B-field pressure dominates over turbulent and thermal pressures. Thermal pressure might act in the same orientation as those of B-fields to accelerate the expansion of those I-fronts. The observed morphological correspondence among the B-fields, filament, and bipolar bubble demonstrate that the B-fields are important to the cloud contraction that formed the filament, gravitational collapse and star formation in it, and in feedback processes. The latter include the formation and evolution of mid-infrared bubbles by means of B-field supported propagation and expansion of I-fronts. These may shed light on preexisting conditions favoring the formation of the massive stellar cluster in RCW57A.