Polarized CO Emission from Molecular Clouds

Linearly polarized, non-masing, rotational lines have been detected for thefirst time in the interstellar medium. This effect occurs in molecular cloudswith a magnetic field, and traces the field direction, offering an alternativetechnique to dust emission polarimetry. The line polarization mechanism issimilar to that in masers, but with lower degrees of polarization (~1%). Wehave detected the effect towards the Galactic Centre and its surrounding `2 pcring', and in the molecular clouds S140 and DR21 (tentatively), in the lines COJ=2-1 and J=3-2, and 13CO J=2-1. The deduced magnetic field directions agreewell with previous dust polarimetry results, confirming that the linepolarization is a real effect. This new technique will be useful in sourcesthat are too faint for dust polarimetry, and can also be used to investigate3-dimensional morphology of magnetic fields, where the velocity structure ofthe clouds is known.Comment: 15 pages, 1 figure, to be published in ApJ Letter