Magnetic Field Structures in Star Forming Regions: Mid-Infrared Imaging Polarimetry of K3-50

TL;DR

This study uses mid-infrared imaging polarimetry to map magnetic field structures in the star-forming region K3-50, revealing their role in the region's physical dynamics and providing a new application of the Aitken technique.

Contribution

It presents the first spatially-resolved magnetic field mapping in multiple layers using the Aitken polarimetric technique, enhancing understanding of magnetic influence in star formation regions.

Findings

Magnetic fields are potentially dynamically important within 0.2 pc of the molecular torus.

The molecular torus is likely rotationally supported outside the innermost region.

Magnetic fields may help confine the ionised outflow near the cavity wall.

Abstract

We report new imaging polarimetry observations of the Galactic compact HII region K3-50 using CanariCam at the Gran Telescopio Canarias. We use a standard polarimetric analysis technique, first outlined by Aitken, to decompose the observed polarisation images centred at 8.7, 10.3, and 12.5 $\mu$m into the emissive and absorptive components from silicate grains that are aligned with the local magnetic field. These components reveal the spatially-resolved magnetic field structures across the mid-infrared emission area of K3-50. We examine these structures and show that they are consistent with previously observed features and physical models of K3-50, such as the molecular torus and the ionised outflow. We propose a 3D geometry for all the structures seen at different wavelengths. We also compute relevant physical quantities in order to estimate the associated magnetic field strengths that would be implied under various physical assumptions. We compare these results with MHD simulations of protostar formation that predict the magnetic field strength and configuration. We find that the magnetic field may be dynamically important in the innermost 0.2 pc of the molecular torus, but that the torus is more likely to be rotationally-supported against gravity outside this radius. Similarly, magnetic fields are unlikely to dominate the {\em global} physics of the ionised outflow, but they may be important in helping confine the flow near the cavity wall in some locations. Ours is the first application of the Aitken technique to spatially-resolved magnetic field structures in multiple layers along the line of sight, effectively a method of "polarisation tomography."