The role of radiative torques in the molecular cloud core L43

TL;DR

This study investigates how anisotropic radiation influences dust grain alignment via radiative torques in the molecular cloud core L43, using polarization data to infer magnetic field properties and support the radiative torque alignment mechanism.

Contribution

It provides observational evidence supporting radiative torques as the main dust alignment mechanism in molecular clouds, analyzing polarization spectra and magnetic field strengths in L43.

Findings

Magnetic field strengths range from 13 to 60 μG across wavelengths.

Negative slope of polarization spectrum suggests temperature and radiation field influence.

Results align with radiative torque alignment theory.

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 $154 \mu\mathrm{m}$ (SOFIA/HAWC+), as well as $450 \mu\mathrm{m}$ and $850 \mu\mathrm{m}$ (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 $\mu\mathrm{G}$, 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.