Fragmenting Filaments and Evolving Cores -- Insights from Dust Polarisation Study of a filament in Northern Orion B

TL;DR

This study uses 850 μm dust polarization from JCMT POL-2, complemented by Herschel $N_{\mathrm{H2}}$ maps and Planck data, to probe a parsec-scale filament in Orion B and its embedded cores. It finds a magnetically supercritical filament with a mean plane-of-sky field ~31 μG, and a fragmentation sequence into 1 starless, 3 prestellar, and 4 protostellar cores, each showing distinct polarisation fractions and field orientations. Through DCF and compressible-turbulence formalisms, the work reveals a progression from weak to strong magnetic field coupling as cores evolve, reflected in the B–n relation with κ values shifting from ~0.66 for prestellar/starless to ~0.42 for protostellar cores. The results highlight depolarisation mechanisms, core–neighbourhood magnetic-field coherence, and the critical role of magnetic support in filament fragmentation, while acknowledging data limitations and the need for broader, higher-resolution polarimetric surveys. Overall, the paper advances understanding of how magnetic fields influence filament fragmentation and core evolution in star-forming regions. Please note all mathematical expressions are denoted with $...$ in the text above.

Abstract

We present an analysis of polarised dust emission at 850 micron for a parsec long filament in the northern part of the Orion B molecular cloud. The region was observed by the JCMT SCUBA-2/POL-2 polarimeter. The filament has a line mass (~80 Msun/pc) larger than the critical (magnetic) line mass (~37 Msun/pc); and hosts one starless, three prestellar, and four protostellar cores, with masses in the range 0.13 to 9.13 Msun. The mean (debiased) polarisation fraction of the filament and core pixels was calculated to be 5.3+/-0.3% and 3.2+/-0.3%, respectively, likely reflecting their distinct physical conditions. The polarisation fraction for the cores does not depend on the type of core, and was found to decrease with increasing column density, varying from 6-11% at the filament edges to 1$^{+0.7}_{-0.1}$% in the denser parts ($N_{H2}\gtrsim$2x10$^{22}$cm$^{-2}$). Magnetic field orientation of the protostellar cores, in contrast to prestellar cores, appears to be relatively aligned with the magnetic field orientation of the local filament in this region. Using the Davis-Chandrasekhar-Fermi formalism the plane-of-sky magnetic field strength for the protostellar cores (~39-110 microG) was found to be higher than that of the prestellar cores (~22-61 microG); and weakest for the starless core (~6 microG). The average value for the filament was found to be ~31 microG. The magnetic field-volume density relation for the prestellar/starless cores and protostellar cores suggests a transition from weak field case to strong field case as the cores evolve from prestellar to protostellar phase.

Figures (12)

• Figure 1: JCMT stokes I map of the observed region. Contours from the HGBS column density map (resolution$\sim$18.2) are drawn at 0.2, 0.25, 0.55, 0.7, 1, 1.5, 2, and 3$\times$10$^{22}$ cm$^{-2}$. Planck magnetic field half-vectors are shown by magenta line segments. It should be noted that the Planck data is oversampled here. Magenta circles on bottom left show the Planck beam ($\sim$5) and the JCMT beam ($\sim$14). Green box is the filamentary region analysed in this work.
• Figure 2: (top)$PF_{deb}$ image overlaid with HGBS sources. Red, Green, and Blue ellipses are the starless, prestellar, protostellar extended-cores, respectively. We have labelled these cores 1-8 for ease of analyses. Contours mark the $\mathrm{N_{{ \mathrm{H2}}}}$ thresholds of 0.55, 0.7, 1, 1.5, 2, and 3$\times$10$^{22}$ cm$^{-2}$. Blue and yellow line segments denote the orientation of the POL-2 $\mathrm{B_{pos}}$ half-vectors ($\theta$, measured east of north) for the filament and the cores, respectively. Magenta line segments show the magnetic field half-vectors from Planck. It should be noted that the Planck data is oversampled here, and the entire filament is covered by $\sim$3 Planck beams ($\sim$5). Black circle on bottom left shows the JCMT beam ($\sim$14). (bottom) Zoomed-in image of the cores (and their respective neighbourhoods used for calculation in Section \ref{['section_PolParams_CoreRegions']}) overlaid on the column density map. Thicker blue and yellow segments (in each subfigure) denote the mean $\theta$ of the neighbourhood ($\theta$$_{ngbr}$) and the core ($\theta$$_{extCore}$), respectively. These two segments have been scaled by $PF_{deb}$; but for cores 1, 2, and 8, the lengths have been further scaled down by a factor of 3. At the top of each subfigure, mean $PF_{deb}$ values for the core and the neighbourhood (in brackets) are given.
• Figure 3: (a) Polar histogram of $\theta$ (orientation angle of $\mathrm{B_{pos}}$). (b) Histogram of $PF_{deb}$. The distributions for the filament and extended-core regions are shown in blue and yellow, respectively; and the mean values as blue lines (50$\pm$3 degrees and 5.3$\pm$0.3%) and yellow lines (30$\pm$4 degrees and 3.2$\pm$0.3%), respectively. The color spans on either side of the mean vertical lines show the error on the mean.
• Figure 4: Polar histograms for the eight extended-core regions and respective neighbourhood regions are shown in yellow and blue, respectively; and the mean values (see Table \ref{['table_extCoreSummary']}) as yellow and blue lines, respectively. Angles are measured counter-clockwise from 0° at the top.
• Figure 5: (a)$PF_{deb}$ vs. $\theta$ for the eight extended-core regions (filled symbols) and the respective neighbourhoods (empty symbols). The horizontal and vertical extents for each point here show the ($\pm$)standard deviation of the pixels for the region that point represents. Vertical lines at 0 and 180 degrees show the extent of the graph, as 180 degree wraps around to 0 degree. (b) Difference of core values from their respective neighbourhood values. The horizontal and vertical extents for the respective points here are the error bars.