Multiwavelength Polarimetry of the Filamentary Cloud IC5146: II. Magnetic Field Structures

TL;DR

This study reveals the magnetic field structure of the IC5146 cloud using multiwavelength polarimetry, showing magnetic regulation of cloud evolution and a transition from subcritical to supercritical states.

Contribution

It provides detailed magnetic field morphology and strength estimates, demonstrating the magnetic field's role in cloud evolution and star formation in IC5146.

Findings

Magnetic fields are well-ordered and aligned with filament structures.

Magnetic field strength scales with density as a power-law with index ~0.5.

Mass-to-flux ratio varies from subcritical to supercritical across the cloud.

Abstract

The IC5146 cloud is a nearby star-forming region in Cygnus, consisting of molecular gas filaments in a variety of evolutionary stages. We used optical and near-infrared polarization data toward the IC5146 cloud, reported in the first paper of this series, to reveal the magnetic fields in this cloud. Using the newly released $Gaia$ data, we found that the IC5146 cloud may contain two separate clouds: a first cloud, including the densest main filament at a distance of $\sim$600 pc, and a second cloud, associated with the Cocoon Nebula at a distance of $\sim$800 pc. The spatially averaged H-band polarization map revealed a well-ordered magnetic field morphology, with the polarization segments perpendicular to the main filament but parallel to the nearby sub-filaments, consistent with models assuming that the magnetic field is regulating cloud evolution. We estimated the magnetic field strength using the Davis-Chandrasekhar-Fermi method, and found that the magnetic field strength scales with volume density with a power-law index of $\sim0.5$ in the density range from $N_{H_2}\sim$ 10 to 3000 cm$^{-3}$, which indicates an anisotropic cloud contraction with a preferred direction along the magnetic field. In addition, the mass-to-flux ratio of the cloud gradually changes from subcritical to supercritical from the cloud envelope to the deep regions. These features are consistent with strong magnetic field star-formation models and suggest that the magnetic field is important in regulating the evolution of the IC5146 cloud.