Evolution of magnetized hub-filament systems: Comparing the observed properties of W3(OH), W3 Main, and S 106

TL;DR

The study analyzes three hub-filament systems (W3(OH), W3 Main, S 106) at successive evolutionary stages to understand how magnetic fields and mass assembly evolve in cluster-forming hubs. Using new JCMT/SCUBA-2/POL-2 850 µm polarization maps alongside archival infrared and radio data, the authors map column densities, derive hub centers, and construct filament line-mass maps to compare hub and filament properties. They observe systematic changes in filament line-mass distributions, polarization fractions, and B-field–filament orientations as evolution proceeds, with outflows and radiation bubbles reshaping the plane-of-sky magnetic fields and aligning them with cavity walls. A key finding is the pervasive occurrence of two adjacent star-formation nodes within hubs (double-nodes), a pattern that persists across the sample and may reflect sequential star formation within evolving hubs. The results imply that magnetic fields remain dynamically important in hub formation, with hub sizes increasing over time and magnetic-field geometry progressively dragged toward alignment with high-density filaments feeding the hub, thereby influencing mass accretion and fragmentation in massive star formation.

Abstract

In this study, we examine three cluster-forming hub-filament systems (HFS) - W3(OH), W3 Main, and S 106 - spanning evolutionary stages from early to advanced, with a focus on their magnetic field (B-field) structures and filament line-mass distributions. Our goal is to identify indicators of HFS evolution, particularly within their hubs, as star formation progresses. Our analysis combines observations of dense star-forming gas and young stellar populations. We present new JCMT/POL-2 observations of 850micron dust polarized emission to probe magnetic field morphology and dense gas structures. Archival infrared and radio data are also used to trace star formation activity. We derive radial column density profiles centered on the hubs to define distinct filament and hub regions. We then analyze histograms of line mass, polarization intensity (PI), polarization fraction (PF), and the relative orientation between B-fields and filaments. As HFS evolve, we observe changes in the filament line-mass function (FLMF), PF, and B-field-filament alignment - especially within the hub, which also increases in size. Massive bipolar outflows and radiation bubbles reshape the plane-of-sky B-fields, aligning them with cavity walls and shells, consistent with known rearrangements near HII regions. We also find a notable similarity between hub sizes and young cluster radii. "Double-node" star formation - where two subregions within a hub show different evolutionary stages - emerges as a common HFS feature. We present evidence for its widespread occurrence across several well-studied, nearby star-forming clouds.

Figures (17)

• Figure 1: Left: Total intensity Stokes $I$ map at 850 $\mu$m observed with the JCMT SCUBA-2/POL-2 towards W3(OH) in unit of mJy beam$^{-1}$. The contours are $I=30, 100, 500, 1000$, and $5000\,$mJy beam$^{-1}$. The lowest contour of $I=30\,$mJy beam$^{-1}$ is equivalent to $I/\delta I\sim 13$. The red lines show the orientation of the POS B-field angle ($\chi_{B_{\rm POS}}$) for $I/\delta I>5$, $PI/\delta PI>1$, and $PF<25\%$. The lengths of the red lines are proportional to $PF$. A line showing $PF=10\%$ is indicated on the plot. The data are at an angular resolution of $14\arcsec$ or $\sim0.14$ pc at the 2 kpc distance of the source. Right: The contours and red lines are the same as in the left panel. The red lines are plotted with the same length to better show the B-field orientation. The background is a color image composed using Spitzer 3.6 $\mu\rm m$ , 4.5 $\mu\rm m$ , and 8.0 $\mu\rm m$ images for the blue, green, and red, respectively.
• Figure 2: Same as Fig. \ref{['W3OH-I-Bfield-Spitzer']} for W3-Main. The data are at an angular resolution of $14\arcsec$ or $\sim0.14$ pc at the 2 kpc distance of the source.
• Figure 3: Same as Fig. \ref{['W3OH-I-Bfield-Spitzer']} for S106. The data are at an angular resolution of $14\arcsec$ or $\sim0.09$ pc at the 1.3 kpc distance of the source.
• Figure 4: Left: Blow-up image of the 850 $\mu$m Stokes $I$ emission towards the hub of W3 Main (in units of Jy/beam), overlaid by normalized POS B-field lines. Grey contours display VLA 6 cm emission at 0.1 resolution, the symbols A, B, C, and D mark the known compact HII regions. The youngest most luminous source IRS 5 is also indicated. The white arcs aid to visualize the organized B-field pattern. The white star and arrow indicate the center of a CO outflow and its direction, respectively (see text for details). Right: W3 foreground mini-HFS, also referred to as W3 Main SE indicated with the square on the left panel. The Spitzer three color image is overplotted with Stokes I contours and normalized POS B-field line.
• Figure 5: Hub regions of W3(OH), W3-Main, and S106 (clockwise from top left). The color compositions are the same as the right panels of Figs. \ref{['W3OH-I-Bfield-Spitzer']}, \ref{['W3Main-I-Bfield-Spitzer']}, and \ref{['S106-I-Bfield-Spitzer']}. Here the cyan contours trace column densities of 10$^{22}$, 10$^{23}$ and 5$\times$10$^{23}$ cm$^{-2}$. The black ellipses represent the hubs as described in Table. \ref{['table1']}, and their centers are marked with a black box-circle symbol. The blue contours represent VLA 4.89 GHz continuum emission obtained in B/C configurations. This emission is shown with a beam size of 0.7 for W3(OH), however, it is compact at the scale of 0.08. In the "two node" system, the OpCl* Carpenter2000 represents the more evolved node compared to the younger node composed of W3(H$_2$O) and W3(OH) Qin2016Wyrowski1999. In W3-Main, IRS 5 surrounded by compact HII regions Tieftrunk1997 A & B represent the younger node, while the evolved node corresponds to the compact regions C & D. In S 106, the younger and older nodes correspond to IRS 4 and the infrared cluster, respectively Saito2009.