On interstellar light polarization by diamagnetic silicate and carbon dust in the infrared

TL;DR

This study models the behavior of diamagnetic interstellar dust grains in magnetic fields, showing how their alignment affects infrared light polarization and matches astronomical observations, with implications for understanding interstellar dust properties.

Contribution

It provides a numerical analysis of dust grain dynamics, revealing persistent partial alignment and its impact on infrared polarization, supported by detailed case studies of silicate grains.

Findings

Partial alignment persists in most cases, affecting polarization.

The model matches observed infrared polarization levels.

Phonons influence long-wavelength polarization and extinction.

Abstract

The motion of diamagnetic dust particles in interstellar magnetic fields is studied numerically with several different sets of parameters. Two types of behavior are observed, depending on the value of the critical number $R$, which is a function of the grain inertia, the magnetic susceptibility of the material and of the strength of rotation braking. If $R\leq10$, the grain ends up in a static state and perfectly aligned with the magnetic field, after a few braking times. If not, it goes on precessing and nutating about the field vector for a much longer time. Usual parameters are such that the first situation can hardly be observed. Fortunately, in the second and more likely situation, there remains a persistent partial alignment which is far from negligible, although it decreases as the field decreases and as $R$ increases. The solution of the complete equations of motion of grains in a field helps understanding the details of this behavior. One particular case of an ellipsoidal forsterite silicate grain is studied in detail and shown to polarize light in agreement with astronomical measurements of absolute polarization in the infrared. Phonons are shown to contribute to the progressive flattening of extinction and polarization towards long wavelengths. The measured dielectric properties of forsterite qualitatively fit the Serkowski peak in the visible.