Understanding the Links among the Magnetic Fields, Filament, Bipolar Bubble, and Star Formation in RCW 57A Using NIR Polarimetry

Chakali Eswaraiah,Y Nakajima,Wen-Ping Chen,Shih-Ping Lai,M Tamura,Jungmi Kwon,A M Magalhães,S Nishiyama,Jia-Wei Wang,S Sharma,R Purcell,G Maheswar,A K Pandey

doi:10.3847/1538-4357/aa917e

Abstract

Abstract The influence of magnetic fields (B-fields) on 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 the expansion and propagation of I-fronts forming a bipolar bubble. We present results based on near-infrared polarimetric observations toward the central ∼8′ × 8′ area of the star-forming region RCW 57A, 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 perpendicularly to the filament long axis. The B-fields exhibit an hourglass morphology that closely follows the structure of the bipolar bubble. The mean B-field strength, estimated using the Chandrasekhar–Fermi method (CF method), is 91 ± 8 μG. B-field pressure dominates over turbulent and thermal pressures. Thermal pressure might act in the same orientation as the 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, to the gravitational collapse and star formation in it, and in feedback processes. The last one includes 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.

Highlights

Massive stars (> 8M ) have profound effects on their surrounding natal cloud material through driving strong ionizing radiation, powerful stellar winds, outflows, expanding Hii regions, and supernova explosions which either trigger, or halt, star formation (Beuther et al 2002; Zinnecker & Yorke 2007; Lee & Chen 2007; Smith et al 2010; Roccatagliata et al 2013)
Star forming regions associated with Hii regions exhibit spectacular morphologies, such as spherical, or ring-like bubbles, and bipolar or unipolar bubbles (Churchwell et al 2006, 2007; Deharveng et al 2010; Simpson et al 2012) that may be surrounded by bright rimmed clouds (BRCs) (Sugitani et al 1991; Sugitani & Ogura 1994)
Though bipolar bubbles are the natural outcome of anisotropic expansion of ionizing fronts from Hii regions hosting massive O/B-type star(s) located in filaments (Minier et al 2013; Deharveng et al 2015; Zhang et al 2016), details involved in their formation and evolution processes remain poorly understood

Summary

Introduction

Massive stars (> 8M ) have profound effects on their surrounding natal cloud material through driving strong ionizing radiation, powerful stellar winds, outflows, expanding Hii regions, and supernova explosions which either trigger, or halt, star formation (Beuther et al 2002; Zinnecker & Yorke 2007; Lee & Chen 2007; Smith et al 2010; Roccatagliata et al 2013). According to 2D (Bodenheimer et al 1979) and 3D (Fukuda & Hanawa 2000) hydrodynamic simulations, the evolution of an Hii region in a filamentary cloud induces supersonic and subsonic I-fronts along the minor and major axes of the filament, respectively. This results the distribution of low and high density ma-

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