The Optical Extinction Law Depends on Magnetic Field Orientation: The $R_V$-$\psi$ Relation

TL;DR

This paper shows that the optical extinction law, characterized by R_V, varies with magnetic field orientation due to dust grain alignment, explaining observed large-scale R_V variations in the Galaxy.

Contribution

It introduces a model linking magnetic field orientation to R_V variations, providing a physical explanation for observed extinction law changes.

Findings

R_V varies from 3.21 to 3.05 with magnetic field orientation

Magnetic field orientation accounts for large-scale R_V variation

Aligned dust grains influence wavelength-dependent extinction

Abstract

For aspherical interstellar dust grains aligned with their short axes preferentially parallel to the local magnetic field, the amount of extinction per grain is larger when the magnetic field is along the line of sight and smaller when in the plane of the sky. To the extent that optical extinction arises from both aligned and unaligned grain populations with different extinction properties, changes in the magnetic field orientation induces changes in its wavelength dependence, parameterized by $R_V \equiv A_V/E(B-V)$. We demonstrate that the measured total and polarized extinction curves of the diffuse Galactic interstellar medium imply $R_V$ varies from 3.21 when the magnetic field is along the line of sight ($\psi = 0$) to $R_V = 3.05$ when in the plane of the sky ($\psi = 90^\circ$). This effect could therefore account for much of the large-scale $R_V$ variation observed across the sky ($\sigma(R_V) \simeq 0.2$), particularly at high Galactic latitudes.