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 H ii 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 \arcmin \times 8 \arcmin area of the star forming region RCW57A which hosts an H ii region , a filament , and a bipolar bubble .
Polarization measurements of 178 reddened background stars , out of the 919 detected sources in the JHK _ { s } -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 RCW 57A .