Non-Zeeman Circular Polarization of CO rotational lines in SNR IC 443
  Context: We investigate non-Zeeman circular polarization and linear
polarization levels of up to 1% of $^{12}$CO spectral line emission detected in
a shocked molecular clump around the supernova remnant (SNR) IC 443, with the
goal of understanding the magnetic field structure in this source. Aims: We
examine our polarization results to confirm that the circular polarization
signal in CO lines is caused by a conversion of linear to circular
polarization, consistent with anisotropic resonant scattering. In this process
background linearly polarized CO emission interacts with similar foreground
molecules aligned with the ambient magnetic field and scatters at a transition
frequency. The difference in phase shift between the orthogonally polarized
components of this scattered emission can cause a transformation of linear to
circular polarization. Methods: We compared linear polarization maps from dust
continuum, obtained with PolKa at APEX, and $^{12}$CO ($J=2\rightarrow1$) and
($J=1\rightarrow0$) from the IRAM 30-m telescope and found no consistency
between the two sets of polarization maps. We then reinserted the measured
circular polarization signal in the CO lines across the source to the
corresponding linear polarization signal to test whether before this linear to
circular polarization conversion the linear polarization vectors of the CO maps
were aligned with those of the dust. Results: After the flux correction for the
two transitions of the CO spectral lines, the new polarization vectors for both
CO transitions aligned with the dust polarization vectors, establishing that
the non-Zeeman CO circular polarization is due to a linear to circular
polarization conversion.
