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Context: We investigate non-Zeeman circular polarization and linearpolarization levels of up to 1% of $^{12}$CO spectral line emission detected ina shocked molecular clump around the supernova remnant (SNR) IC 443, with thegoal of understanding the magnetic field structure in this source. Aims: Weexamine our polarization results to confirm that the circular polarizationsignal in CO lines is caused by a conversion of linear to circularpolarization, consistent with anisotropic resonant scattering. In this processbackground linearly polarized CO emission interacts with similar foregroundmolecules aligned with the ambient magnetic field and scatters at a transitionfrequency. The difference in phase shift between the orthogonally polarizedcomponents of this scattered emission can cause a transformation of linear tocircular polarization. Methods: We compared linear polarization maps from dustcontinuum, obtained with PolKa at APEX, and $^{12}$CO ($J=2\rightarrow1$) and($J=1\rightarrow0$) from the IRAM 30-m telescope and found no consistencybetween the two sets of polarization maps. We then reinserted the measuredcircular polarization signal in the CO lines across the source to thecorresponding linear polarization signal to test whether before this linear tocircular polarization conversion the linear polarization vectors of the CO mapswere aligned with those of the dust. Results: After the flux correction for thetwo transitions of the CO spectral lines, the new polarization vectors for bothCO transitions aligned with the dust polarization vectors, establishing thatthe non-Zeeman CO circular polarization is due to a linear to circularpolarization conversion.

Non-Zeeman Circular Polarization of CO rotational lines in SNR IC 443