Polarized CO Emission from Molecular Clouds

Linearly polarized, nonmasing, rotational lines have been detected for the first time in the interstellar medium. This effect occurs in molecular clouds with a magnetic field and traces the field direction, offering an alternative technique to dust emission polarimetry. The line polarization mechanism is similar to that in masers, but with lower degrees of polarization (~1%). We have detected the effect toward the Galactic center and its surrounding "2 pc ring" and in the molecular clouds S140 and DR 21 (tentatively), in the lines CO J=2-1 and J=3-2 and 13CO J=2-1. The deduced magnetic field directions agree well with previous dust polarimetry results, confirming that the line polarization is a real effect. This new technique will be useful in sources that are too faint for dust polarimetry and can also be used to investigate the three-dimensional morphology of magnetic fields in cases in which the velocity structure of the clouds is known.