Non-Zeeman Circular Polarization of Molecular Rotational Spectral Lines

TL;DR

This paper reports the detection of circular polarization in molecular rotational lines in Orion KL, proposes a resonant scattering model to explain it, and suggests a new method to measure magnetic field components using circular polarization.

Contribution

It introduces a physical model based on resonant scattering to explain non-Zeeman circular polarization in molecular lines, enabling magnetic field measurements without Zeeman splitting.

Findings

Detected 1-2% circular polarization in 12CO lines.

Proposed a resonant scattering model explaining polarization.

Showed polarization proportional to magnetic field squared.

Abstract

We present measurements of circular polarization from rotational spectral lines of molecular species in Orion KL, most notably 12CO (J=2 - 1), obtained at the Caltech Submillimeter Observatory with the Four-Stokes-Parameter Spectra Line Polarimeter. We find levels of polarization of up to 1 to 2% in general, for 12CO (J=2 - 1) this level is comparable to that of linear polarization also measured for that line. We present a physical model based on resonant scattering in an attempt to explain our observations. We discuss how slight differences in scattering amplitudes for radiation polarized parallel and perpendicular to the ambient magnetic field, responsible for the alignment of the scattering molecules, can lead to the observed circular polarization. We also show that the effect is proportional to the square of the magnitude of the plane of the sky component of the magnetic field, and therefore opens up the possibility of measuring this parameter from circular polarization measurements of Zeeman insensitive molecules.