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Molecular line polarization is a unique source of information about themagnetic fields and anisotropies in the circumstellar envelopes of evolvedstars. Here we present the first detection of thermal CO(J=2-1) and SiO(J=5-4,v=0) polarization, in the envelope of the asymptotic giant branch star IK Tau.The observed polarization direction does not match predictions forcircumstellar envelope polarization induced only by an anisotropic radiationfield. Assuming that the polarization is purely due to the Goldreich-Kylafiseffect, the linear polarization direction is defined by the magnetic field aseven the small Zeeman splitting of CO and SiO dominates the molecularcollisional and spontaneous emission rates. The polarization was mapped usingthe Submillimeter Array (SMA) and is predominantly north-south. There is closeagreement between the CO and SiO observations, even though the CO polarizationarises in the circumstellar envelope at ~800 AU and the SiO polarization at<250 AU. If the polarization indeed traces the magnetic field, we can thusconclude that it maintains a large-scale structure throughout the circumstellarenvelope. We propose that the magnetic field, oriented either east-west ornorth-south is responsible for the east-west elongation of the CO distributionand asymmetries in the dust envelope. In the future, the Atacama LargeMillimeter/submillimeter Array will be able to map the magnetic field using COpolarization for a large number of evolved stars.

Polarization of thermal molecular lines in the envelope of IK Tau