The role of radiative torques in the molecular cloud core L43

TL;DR

The study analyzes dust grain alignment in the molecular cloud core L43 by combining multiwavelength polarization maps with radiative-transfer modeling and a differential-measure analysis (DMA) to assess the role of radiative torque (RAT) alignment. By exploiting the embedded YSO as an anisotropic radiation source and using data at $154$, $450$, and $850\,\mu$m, the authors derive plane-of-sky magnetic-field strengths in the range $B_{\rm POS} \sim 13$–$60\,\mu$G and observe a negative slope in the polarization spectrum, with LOS effects and dust-property variations contributing to the interpretation. POLARIS simulations show that the heterogeneity of dust temperatures along the line of sight, along with magnetic-field orientation, can reproduce part of the observed spectral behavior, supporting RAT as the dominant alignment mechanism in this region. The results demonstrate that polarization carries information about the local radiation field and temperature, with implications for inferring 3D magnetic-field structure in star-forming regions from multiwavelength polarimetry.

Abstract

Polarized emission from interstellar dust grains is commonly used to infer information about the underlying magnetic field from the diffuse interstellar medium to molecular cloud cores. Therefore, the ability to accurately determine properties of the magnetic field requires a thorough understanding of the dust alignment mechanism. We investigate the influence of anisotropic radiation fields on the alignment of dust particles by magnetic fields, known as radiative torque (RAT) alignment. Specifically, we take advantage of the unique spatial configuration of the molecular cloud core L43, which contains an embedded yet optically visible star acting as a local source of anisotropic illumination. Based on polarization maps obtained at wavelengths of $154 μ\mathrm{m}$ (SOFIA/HAWC+), as well as $450 μ\mathrm{m}$ and $850 μ\mathrm{m}$ (JCMT/SCUBA-2), which show variations in the degree and angle of polarized emission across all wavelengths, we applied the differential measure analysis method to infer magnetic field strengths and analyze the global polarization spectrum of this source. We derived plane-of-sky magnetic field strengths ranging from approximately 13 to 60 $μ\mathrm{G}$, varying with wavelength, and find a negative slope of the polarization spectrum. Compared to 3D radiative transfer simulations, this finding can be attributed, at least partially, to variations in dust properties and temperatures along the line of sight. However, the additional influence of variations in the magnetic field orientation along the line of sight cannot be ruled out. Our results favor radiative torques as the primary alignment mechanism, as they indicate that the degree of polarization is dependent on temperature and hence the strength of the local radiation field.

Figures (12)

• Figure 1: Dust continuum maps of the L43 region. Black lines indicate the polarization vectors. The vectors are rotated by $90^{\circ}$ to indicate the magnetic field direction and are binned to the same $12^{\prime\prime}$ grid. Black contour lines mark regions where the flux has doubled. White dashed ellipses show the two regions of interest that are referred to throughout this work. The effective beam size of each instrument is indicated with a red circle. Left: $154\,\mu\mathrm{m}$ continuum emission (SOFIA/HAWC+). Top right: $450\,\mu\mathrm{m}$ continuum emission (SCUBA-2/POL-2). Bottom right: $850\,\mu\mathrm{m}$ continuum emission (SCUBA-2/POL-2).
• Figure 2: Degree of polarization as a function of the normalized intensity in the RNO 91 region (top panel) and the L43E region (bottom panel). The black lines indicate the best fit of Eq. \ref{['PvsIEq']} to each dataset. In the L43E region, we find $a_2 = 1.03^{0.10}_{-0.10}$ ($\chi^2_{\mathrm{red}}=0.63$) at $154\,\mu\mathrm{m}$, $a_2 = 0.81^{0.08}_{-0.07}$ ($\chi^2_{\mathrm{red}}=0.73$) at $450\,\mu\mathrm{m}$ and $a_2 = 0.97^{0.05}_{-0.05}$ ($\chi^2_{\mathrm{red}}=0.99$) at $850\,\mu\mathrm{m}$. In the RNO 91 region, we find $a_2 = 0.82^{0.02}_{-0.02}$ ($\chi^2_{\mathrm{red}}=1.79$) at $154\,\mu\mathrm{m}$, $a_2 = 1.04^{0.10}_{-0.10}$ ($\chi^2_{\mathrm{red}}=0.73$) at $450\,\mu\mathrm{m}$ and $a_2 = 1.02^{0.09}_{-0.09}$ ($\chi^2_{\mathrm{red}}=0.69$) at $850\,\mu\mathrm{m}$.
• Figure 3: Global polarization spectra for the entire region (left panel), the L43E region (center panel), and the RNO 91 region (right panel). Points indicate the median, and error bars the MAD at a given wavelength. The spectra are normalized at $\lambda_0 = 450\,\mu\mathrm{m}$. A black dashed line indicates unity.
• Figure 4: Spatially resolved column density (top panel) and LOS temperature (bottom panel) distributions, acquired by fitting Eq. \ref{['BbfitEq']} to Herschel PACS/SPIRE and JCMT/SCUBA-2 data at observing wavelengths of $160\,\mu\mathrm{m}$, $250\,\mu\mathrm{m}$, $350\,\mu\mathrm{m}$, $500\,\mu\mathrm{m}$, and $850\,\mu\mathrm{m}$. Points where $\Delta_{T_\mathrm{d}}>5\,\mathrm{K}$ and $N_{\mathrm{H_2}}/\Delta_{N_{\mathrm{H_2}}}<1$ are excluded, where $\Delta_{T_\mathrm{d}}$ and $\Delta_{N_\mathrm{H_2}}$ refer to the errors on the respective fit parameter. Overlaid are contour lines of the $850\,\mu\mathrm{m}$ continuum emission from Fig. \ref{['FluxMapFig']}.
• Figure 5: POS magnetic field strength $B_\mathrm{POS, \gamma = \pi/4}$ as a function of the separation distance $\ell$ for the L43E (solid lines) and RNO 91 (dashed lines) regions. Polarization measurements are taken at 154$\,\mu\mathrm{m}$ (blue), 450$\,\mu\mathrm{m}$ (green) and 850$\,\mu\mathrm{m}$ (red).