Molecular line polarization is a unique source of information about the magnetic fields and anisotropies in the circumstellar envelopes of evolved stars .
Here we present the first detection of thermal CO ( J = 2 \rightarrow 1 ) and SiO ( J = 5 \rightarrow 4 , \nu = 0 ) polarization , in the envelope of the asymptotic giant branch star IK Tau .
The observed polarization direction does not match predictions for circumstellar envelope polarization induced only by an anisotropic radiation field .
Assuming that the polarization is purely due to the Goldreich-Kylafis effect , the linear polarization direction is defined by the magnetic field as even the small Zeeman splitting of CO and SiO dominates the molecular collisional and spontaneous emission rates .
The polarization was mapped using the Submillimeter Array ( SMA ) and is predominantly north-south .
There is close agreement between the CO and SiO observations , even though the CO polarization arises in the circumstellar envelope at \sim 800 AU and the SiO polarization at \lesssim 250 AU .
If the polarization indeed traces the magnetic field , we can thus conclude that it maintains a large-scale structure throughout the circumstellar envelope .
We propose that the magnetic field , oriented either east-west or north-south is responsible for the east-west elongation of the CO distribution and asymmetries in the dust envelope .
In the future , the Atacama Large Millimeter/submillimeter Array will be able to map the magnetic field using CO polarization for a large number of evolved stars .