The Role of Magnetic Fields in Triggered Star Formation of RCW 120

TL;DR

This study uses near-infrared polarimetric observations to map the magnetic field of RCW 120, revealing its influence on the region's morphology, dynamics, and star formation processes, and highlighting the magnetic field's role in modulating triggered star formation.

Contribution

First measurement of RCW 120's magnetic field via polarimetry, showing its alignment, strength, and impact on star formation and shell dynamics, providing new insights into magnetic influence on triggered star formation.

Findings

Magnetic field aligned with Galactic plane at 20° with a strength of 100±26 μG.

Magnetic field compression observed around the eastern shell.

Magnetic field reduces star formation efficiency compared to hydrodynamic models.

Abstract

We report on the near-infrared polarimetric observations of RCW 120 with the 1.4 m IRSF telescope. The starlight polarization of the background stars reveals for the first time the magnetic field of RCW 120. The global magnetic field of RCW 120 is along the direction of $20^\circ$, parallel to the Galactic plane. The field strength on the plane of the sky is $100\pm26\,\mu$G. The magnetic field around the eastern shell shows evidence of compression by the HII region. The external pressure (turbulent pressure + magnetic pressure) and the gas density of the ambient cloud are minimum along the direction where RCW 120 breaks out, which explains the observed elongation of RCW 120. The dynamical age of RCW 120, depending on the magnetic field strength, is $\sim\,1.6\,\mathrm{Myr}$ for field strength of $100\,\mu$G, older than the hydrodynamic estimates. In direction perpendicular to the magnetic field, the density contrast of the western shell is greatly reduced by the strong magnetic field. The strong magnetic field in general reduces the efficiency of triggered star formation, in comparison with the hydrodynamic estimates. Triggered star formation via the "collect and collapse" mechanism could occur in the direction along the magnetic field. Core formation efficiency (CFE) is found to be higher in the southern and eastern shells of RCW 120 than in the infrared dark cloud receiving little influence from the HII region, suggesting increase in the CFE related to triggering from ionization feedback.