The generation and transformation of polarisation signals in molecular lines through collective anisotropic resonant scattering

TL;DR

This paper explores how anisotropic resonant scattering (ARS) in molecular lines can generate and transform polarisation signals, including circular and elliptical polarisation, influenced by magnetic fields, with implications for astronomical observations.

Contribution

It introduces a collective ARS model explaining the generation of elliptical polarisation and Faraday effects in molecular spectral lines, extending previous understanding.

Findings

ARS can convert linear to circular polarisation in molecular lines.

Elliptical polarisation depends on magnetic field strength and orientation.

ARS effects are likely observable across various molecules and transitions.

Abstract

We discuss the existence of elliptical polarisation in rotational spectral lines of CO and other molecules within the context of the Anisotropic Resonant Scattering (ARS) model. We show that the effect of ARS on the radiation field can lead to not only the previously predicted transformation of background linear polarisation into circular polarisation (i.e., Faraday conversion), but also the occurrence of Faraday rotation and the generation of elliptically polarised signals in an otherwise initially unpolarised radiation field. This is due to a collective behaviour between the large number of molecules acting as a diffraction ensemble that strongly favours forward scattering over any other mode. Our application to astronomical data demonstrates the dependency of the Stokes parameters on the strength and orientation of the ambient magnetic field, and suggests that ARS will manifest itself for a wide range of molecular species and transitions.