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

Abstract

We present an analysis of polarimetric observations of the three semiregular variables RT Vir, R Crt and W Hya obtained in the 1665 and 1667 MHz OH maser lines. Circular polarisation data were taken at intervals over ap eriod 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 proles. It decreased at velocities where strong OH total flux density was observed, most likely due to blending eects. Individual circularly polarised features exhibited various types of changes; some features were transient or showed signicant 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 RT Vir implies a well-aligned circumstellar magnetic eld. Linearly polarised features preferentially appeared at blue- shifted velocities where they are not suppressed by Faraday rotation. In the blue-shifted 1667 MHz features of RT Vir 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 eect 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 eects in the circumstellar magnetic eld and by global magnetic eld reversal.