Non-Zeeman circular polarization of CO rotational lines in SNR IC 443

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: thezareh@mpifr-bonn.mpg.de
² Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
³ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁴ LERMA, UMR 8112 CNRS, Observatoire de Paris, École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
⁵ European Southern Observatory and Joint ALMA Observatory, Chile

Received: 15 May 2013
Accepted: 8 August 2013

Abstract

Context. We study interstellar magnetic fields by measuring the polarization in molecular spectral lines and thermal emission of dust.

Aims. We report detection of non-Zeeman circular polarization and linear polarization levels of up to 1% in the ¹²CO spectral line emission in a shocked molecular clump around the supernova remnant (SNR) IC 443. 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 the dust continuum, which were obtained with PolKa at APEX, and ¹²CO (J = 2 → 1) and (J = 1 → 0) from the IRAM 30-m telescope. We 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 the linear polarization vectors of the CO maps were aligned with those of the dust before this linear to circular polarization conversion.

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.