Polarization Properties and Magnetic Field Structures in the High-Mass Star-Forming Region W51 Observed with ALMA

TL;DR

This study uses ALMA dust polarization observations to reveal detailed magnetic field structures and their potential role in high-mass star formation within the W51 region, showing organized morphologies and local variations in magnetic support.

Contribution

First high-resolution ALMA polarization observations of W51 high-mass star-forming regions, unveiling complex magnetic field morphologies and introducing a new measure of magnetic influence on gravity.

Findings

Detected polarized emission with levels between 0.1% and 10%.

Revealed organized and complex magnetic field morphologies, including cometary and converging structures.

Identified regions where magnetic fields can effectively oppose gravity or allow collapse.

Abstract

We present the first ALMA dust polarization observations towards the high-mass star-forming regions W51 e2, e8, and W51 North in Band 6 (230 GHz) with a resolution around 0.26" ($\sim5$mpc). Polarized emission in all three sources is clearly detected and resolved. Measured relative polarization levels are between 0.1\% and 10\%. While the absolute polarization shows complicated structures, the relative polarization displays the typical anti-correlation with Stokes $I$, though with a large scatter. Inferred magnetic (B) field morphologies are organized and connected. Detailed substructures are resolved, revealing new features such as cometary-shaped B-field morphologies in satellite cores, symmetrically converging B-field zones, and possibly streamlined morphologies. The local B-field dispersion shows some anti-correlation with the relative polarization. Moreover, lowest polarization percentages together with largest dispersions coincide with B-field convergence zones. We put forward $\sin\omega$, where $\omega$ is the measurable angle between a local B-field orientation and local gravity, as a measure of how effectively the B-field can oppose gravity. Maps of $\sin\omega$ for all three sources show organized structures that suggest a locally varying role of the B-field, with some regions where gravity can largely act unaffectedly, possibly in a network of narrow magnetic channels, and other regions where the B-field can work maximally against gravity.