The role of radiative torques in the molecular cloud core L43

University of Kiel, Institute of Theoretical Physics and Astrophysics, Leibnizstrasse 15, 24118 Kiel, Germany

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Received: 22 September 2025
Accepted: 26 November 2025

Abstract

Context. 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.

Aims. 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.

Methods. Based on polarization maps obtained at wavelengths of 154 μm (SOFIA/HAWC+), as well as 450 μm and 850 μm (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.

Results. We derived plane-of-sky magnetic field strengths ranging from approximately 13-60 μG, 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.

Conclusions. 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.