Probing 3D magnetic fields using starlight polarization and grain alignment theory

TL;DR

This paper introduces a novel method to determine three-dimensional magnetic fields in space by analyzing starlight polarization, leveraging grain alignment theory and synthetic observations to achieve accurate inclination angle measurements.

Contribution

The paper presents a new technique combining starlight polarization efficiency, MRAT alignment theory, and B-field tangling effects to infer 3D magnetic field orientations from observational data.

Findings

Accurate B-field inclination angles can be derived from optical polarization in low-density regions.

Near-infrared polarization extends the technique to high-density regions with reduced grain alignment.

The method enables comprehensive 3D magnetic field mapping in various interstellar environments.

Abstract

Polarization of starlight induced by dust grains aligned with the magnetic field (hereafter B-field) is widely used to measure the two-dimensional B-fields projected onto the plane-of-sky. Here, we introduce a new method to infer three-dimensional B-fields using starlight polarization. We show that the inclination angle or line-of-sight (LOS) component of B-fields can be constrained by the starlight polarization efficiency from observations, the alignment degree provided by the magnetically enhanced radiative torque (MRAT) alignment theory, and the effect of B-field tangling. We first perform synthetic observations of starlight polarization of magnetohydrodynamic (MHD) simulations of a filamentary cloud with our updated POLARIS code incorporating the modern MRAT theory. We test the new technique with synthetic observations and find that the B-field inclination angles can be accurately determined by the synthetic starlight polarization efficiency once the effects of grain alignment, dust properties, and B-field fluctuations are well characterized. The technique can provide an accurate constraint on B-field inclination angles using optical polarization in low-density regions $A_{\rm V}< 3$ with efficient MRAT alignment, whereas the technique can infer further to high-density regions with significant alignment loss at $A_{\rm V} \sim 8 - 30$ by using near-infrared polarization. Our new technique unlocks the full potential of tracing 3D B-fields and constraining dust properties and grain alignment physics on multiple scales of the diffuse interstellar medium and star-forming regions using multi-wavelength starlight polarization observations.