B-field Orion Protostellar Survey (BOPS). IV: The Relative Orientation Between Magnetic Fields and Density Structures in Young Protostellar Envelopes

TL;DR

The study analyzes eight BOPS protostellar envelopes to test how magnetic fields align with density structures on envelope scales of $\sim10^{3}$ au, using the Histogram of Relative Orientations (HRO) applied to column-density maps derived from $870\,\mu$m dust emission. By combining the HRO with the polarization-angle structure function, the authors assess the roles of column density and magnetization in shaping density–field morphology, finding that column density alone does not govern the alignment and that magnetization plays a crucial, source-dependent role. Strongly magnetized envelopes tend toward perpendicular field–density orientations, while weakly magnetized ones show parallel or random alignments, with projection and other effects contributing to scatter. These results imply a coupled influence of density and magnetization on early protoplanetary collapse, and underscore the need for larger samples to confirm statistical trends across envelope scales.

Abstract

We investigate the relative alignment between density structures and magnetic fields in eight young protostars from the ALMA B-field Orion Protostellar Survey. Column density maps are derived from 870 $μ$m dust continuum emission, and the Histogram of Relative Orientations (HRO) method is applied to quantify the correlation between magnetic field orientations and density structures on envelope scales ($\sim$10$^{3}$~au). We find that the relative alignment shows overall weak evidence of systematic evolution with column density, suggesting that column density alone does not fully determine the alignment. The magnetization level also plays a crucial role, with weakly magnetized envelopes exhibiting predominantly parallel or random alignment, whereas strongly magnetized ones show perpendicular configurations even at moderate densities. These results reveal that density and magnetization jointly shape the morphology of protostellar envelopes and the coupling between gravity and magnetic fields during early stages of star formation.

Figures (9)

• Figure 1: ALMA BOPS 870 $\mu$m observations. Left panel: The dust continuum emission (Stokes $I$) is shown in color scales, overlaid with the B-field segments in white and the redshifted and blueshifted outflow directions in red and blue, respectively. Right panel: Column density maps derived from the 870 $\mu$m continuum emission in color scale, overlaid with an LIC texture of the inferred plane-of-sky B-field orientation computed from unit polarization-angle vectors, i.e., the LIC texture encodes direction cabral1993special.
• Figure 2: The relative alignment $\phi$ between the column density contours and the B-field orientation in color scales, overlaid with the column density contours. Contours in each panel start at the 5$\sigma$ ($\sigma$ is the noise level in column density) and increase by factors of 2 (i.e., 1, 2, 4, 8, 16, 32, 64, 128 $\times~ 5\sigma$), with the noise level of $9.9\times10^{21}~{\rm cm^{-2}}$ for HOPS-87, $1.3\times10^{21}~{\rm cm^{-2}}$ for HOPS-182, $5.4\times10^{21}~{\rm cm^{-2}}$ for HOPS-359, $4.7\times10^{21}~{\rm cm^{-2}}$ for HOPS-361, $2.6\times10^{21}~{\rm cm^{-2}}$ for HOPS-370, $2.5\times10^{21}~{\rm cm^{-2}}$ for HOPS-384, $9.7\times10^{21}~{\rm cm^{-2}}$ for HOPS-399, and $7.0\times10^{21}~{\rm cm^{-2}}$ for HOPS-400.
• Figure 3: Histograms of relative orientation (HROs) between the $N_{\rm H_{2}}$ contours and the B-field orientation. For each source, three panels are presented horizontally. Left panel: Global histograms of relative alignment $\phi$, with bin number of 15. Middle panel: HROs for three distinct $N_{\rm H_{2}}$ ranges. The figures present the HROs for the lowest bin, an intermediate bin, and the highest $N_{\rm H_{2}}$ bin (purple, green, and red, respectively). These bins have equal numbers of selected pixels within the indicated $N_{\rm H_{2}}$ ranges. Right panel: Relative orientation parameter $\xi$ calculated for the different $N_{\rm H_{2}}$ bins. The values $\xi>0$ and $\xi<0$ correspond to the B-field being oriented mostly parallel or perpendicular to the $N_{\rm H_{2}}$ contours, respectively. The dashed line is $\xi=0$, which corresponds to the case where there is no preferred relative orientation.
• Figure 4: Structure function of the B-field angles. The horizontal dashed line at $52^\circ$ indicates the expected value for a purely random distribution, and the vertical dashed line indicates the beam size.
• Figure 5: Cumulative distribution functions of the projected relative angles. The colored curves are the observed angle difference between the B-field orientation and the density contours for the eight sources. The black solid line, dashed line, and dotted–dashed line indicate Monte Carlo simulations of the expected projected angle for two vectors that are 3D random (difference angle of two vectors is between $0.0^{\circ}$ and $90.0^{\circ}$), parallel ($0.0^{\circ}-22.5^{\circ}$), and perpendicular ($67.5^{\circ}-90.0^{\circ}$), respectively.