Non-Zeeman Circular Polarization of Molecular Rotational Spectral Lines
  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.
