A. M. Le Squeren

Monitoring of long term behaviour of OH masers in semiregular variables: R Crt, W Hya and RT Vir ?

We present and interpret the results of a long-term OH variability study of three semiregular stars, one type a (SRa), W Hya, and two type b (SRb), R Crt and RT Vir. The 1665 and 1667 MHz OH masers of the three semiregulars were observed at intervals in the period 1982 January{1995 December using the Nan cay radio telescope, and we searched for 1612 MHz emission. The OH maser proles of the studied stars signicantly deviated from a standard double-peaked prole. The timescale of prole changes in the two SRb stars R Crt and RT Vir was as short as a month. The OH proles of the SRa star W Hya were much more stable but since November 1986 a very blue-shifted feature appeared at 1667 MHz. Our phase-lag measurements suggest that this feature comes from a detached OH shell of radius 3 10 16 cm. Faint 1612 MHz emission was found in W Hya only. Weak emission at velocities very close to the systemic velocity usually appeared during some intervals of high maser activity in R Crt and RT Vir and was almost always present in W Hya. For R Crt we estimated that this tangential emission disappeared when the kinetic temperature in the OH maser regions dropped below 150 200 K. For a few features, line narrowing and re-broadening were observed on timescales of 90 200 days. The linewidth was inversely proportional to the peak flux density, suggesting unsaturated amplication. Cyclic variations in the integrated flux density were observed in all the three stars. The OH variability curves were generally characterised by large amplitude (4 6 m ) variations over 400 800 days superimposed with 100 300-day variations of 0:2 2 m . Only the measured OH period of W Hya, of 362 7 days, was in agreement with the optical period. The two SRb stars exhibited multi-periodic OH variability including with two statistically signicant periods. The behaviour of their red- and blue-shifted emission was less correlated than in W Hya. The ratios of the flux densities at 1667 MHz to that at 1665 MHz in all the three stars were about 2 at epochs of high OH activity and usually increased during weak maser emission. Long term behaviour of the OH masers from W Hya resembled that of standard OH Miras, while that of R Crt and RT Vir suggested thin and clumpy envelopes where unsaturated emission was sustained in some clouds.

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

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.

Detection of 1612 MHz OH emission in the semiregular variable stars RT Vir, R Crt and W Hya

We present evidence of 1612 MHz emission in SR variable stars. The two SRb, RT Vir and R Crt, as well as the SRa W Hya have been monitored with the upgraded Nancay radio telescope since February 2001. All three objects have shown a weak 1612 MHz emission occuring in the velocity range of the strongest emission observed in the main-lines. Such a detection is the second observational evidence for emission in the 1612 MHz OH maser satellite line from SRb stars. It also confirms the presence of 1612 MHz emission in the SRa W Hya discovered by Etoka et al. (2001). Such a finding strongly suggests that the shell properties of those three objects are quite similar to those of the Mira stars with similar IR characteristics.