2014MNRAS.443.3819G

We present full-polarization observations, taken with the Parkes radio telescope, of the OH masers at 1665 and 1667 MHz in 34 evolved stars comprising 25 post-Asymptotic Giant Branch (post-AGB) stars and nine high-mass AGB stars (known as LI sources). The main objectives of the present study were to investigate the OH mainline polarization properties of post-AGB stars and high-mass AGB stars and to determine whether there is an association between polarization properties and evolutionary age. Circular and/or linear polarization was detected from 21 of the 34 observed stars. Linear polarization was detected as often as circular polarization, and was relatively more common in the 1667 MHz line. Circular polarization is stronger at 1665 MHz. The massive AGB stars appear to have the strongest magnetic fields. For the post-AGB stars there is a trend for increasing magnetic field strength at the location of the OH mainline masers with increasing post-AGB age. We infer that changes to the stellar magnetic fields play a significant role in the emergence of non-spherical morphologies in post-AGB stars. Six sources show narrow linearly polarized features at 1667 MHz with no corresponding circular polarization at 1667 MHz, that are interpreted as candidate π-components. In one source, a candidate π-component is identified through a Zeeman triplet.