High-Angular Resolution Dust Polarization Measurements: Shaped B-field Lines in the Massive Star Forming Region Orion BN/KL

TL;DR

This study uses high-angular resolution dust polarization observations in Orion BN/KL to reveal a magnetic field with an azimuthally symmetric structure, providing insights into magnetic influence on star formation and outflows.

Contribution

It presents the first high-resolution polarization map of Orion BN/KL, revealing a magnetic field structure with a preferred symmetry plane and azimuthal symmetry, advancing understanding of magnetic roles in massive star formation.

Findings

Magnetic field shows azimuthal symmetry pointing toward explosive outflows.

Polarization pattern indicates magnetic alignment, not mechanical effects from outflows.

Magnetic field has a preferred symmetry plane at P.A. 36 degrees.

Abstract

We present observational results of the thermal dust continuum emission and its linear polarization in one of the nearest massive star-forming sites Orion BN/KL in Orion Molecular Cloud-1. The observations were carried out with the Submillimeter Array. With an angular resolution of 1" (~2 mpc; 480 AU), we have detected and resolved the densest cores near the BN/KL region. At a wavelength of ~870 micron, the polarized dust emission can be used to trace the structure of the magnetic field in this star-forming core. The dust continuum appears to arise from a V-shaped region, with a cavity nearly coincident with the center of the explosive outflows observed on larger scales. The position angles (P.A.s) of the observed polarization vary significantly by a total of about 90 degree but smoothly, i.e., curl-like, across the dust ridges. Such a polarization pattern can be explained with dust grains being magnetically aligned instead of mechanically with outflows, since the latter mechanism would cause the P.A.s to be parallel to the direction of the outflow, i.e., radial-like. The magnetic field projected in the plane of sky is therefore derived by rotating the P.A.s of the polarization by 90 degree. We find an azimuthally symmetric structure in the overall magnetic field morphology, with the field directions pointing toward 2.5" west to the center of the explosive outflows. We also find a preferred symmetry plane at a P.A. of 36 degree, which is perpendicular to the mean magnetic field direction (120 degree) of the 0.5 pc dust ridge. Two possible interpretations of the origin of the observed magnetic field structure are discussed.