* Astronomy

Title: Polarisation of thermal molecular lines in the envelope of IK Tau
Authors: W. H. T. Vlemmings, S. Ramstedt, R. Rao, M. Maercker

Molecular line polarisation 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=2-1) and SiO(J=5-4, v=0) polarisation, in the envelope of the asymptotic giant branch star IK Tau. The observed polarisation direction does not match predictions for circumstellar envelope polarisation induced only by an anisotropic radiation field. Assuming that the polarisation is purely due to the Goldreich-Kylafis effect, the linear polarisation 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 polarisation was mapped using the Submillimetre Array (SMA) and is predominantly north-south. There is close agreement between the CO and SiO observations, even though the CO polarisation arises in the circumstellar envelope at ~800 AU and the SiO polarisation at <250 AU. If the polarisation 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 Millimetre/submillimetre Array will be able to map the magnetic field using CO polarisation for a large number of evolved stars.

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