Variations of polarisation properties of OH maser emission from three semiregular variables

We present an analysis of polarimetric observations of the three semiregular variables RTVir, RCrt and WHya obtained in the 1665 and 1667 MHz OH maser lines. Circular polarisation data were taken at intervals over a period of 10-14 years. During the two last years of the monitoring program the targets were observed at intervals of 3-4 weeks in circular and linear polarisation. Circular polarisation was dominant, whereas linear polarisation, if detected, was weak. The degree of circular polarisation varied considerably across the maser profiles. It decreased at velocities where strong OH total flux density was observed, most likely due to blending effects. Individual circularly polarised features exhibited various types of changes; some features were transient or showed significant variations on timescales of a few weeks to few months. Other features varied slowly over the period of observations of about 5000 days and polarisation reversal occurred. Net circular polarised emission was detected in the all three stars. Variations of net circular polarisation and the degree of circular polarisation of the maser features of RTVir implies a well-aligned circumstellar magnetic field. Linearly polarised features preferentially appeared at blueshifted velocities where they are not suppressed by Faraday rotation. In the blue-shifted 1667 MHz features of RTVir we observed a systematic increase of degree of linear polarisation associated with a gradual decrease in the degree of circular polarisation. This behaviour was possibly linked to variations in the electron density or propagation effect in the maser regions. It is suggested that changes in the observational characteristics of the polarised maser emission of the studied stars can be caused by turbulence effects in the circumstellar magnetic field and by global magnetic field reversal.