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 clou ds with a magnetic field and traces the field direction, offering an altern ative technique to dust emission polarimetry. The line polarization mechani sm is similar to that in masers, but with lower degrees of polarization (si milar to 1%). We have detected the effect toward the Galactic center and it s surrounding "2 pc ring" and in the molecular clouds S140 and DR 21 (tenta tively), in the lines CO J = 2-1 and J = 3-2 and (CO)-C-13 J = 2-1. The ded uced magnetic field directions agree well with previous dust polarimetry re sults, confirming that the line polarization is a real effect. This new tec hnique will be useful in sources that are too Faint fur dust polarimetry an d can also be used to investigate the three-dimensional morphology of magne tic fields in cases in which the velocity structure of the clouds is known.