2008A&A...482..831W

Water vapour maser emission from late-type stars characterises them as asymptotic-giant-branch stars with oxygen-rich chemistry that are losing mass at a substantial rate. Further conclusions on the properties of the stars, however, are hampered by the strong variability of the emission. We wish to understand the reasons for the strong variability of H₂O masers in circumstellar shells of late-type stars. In this paper we study RX Bootis and SV Pegasi as representatives of semiregular variable stars (SRVs). We monitored RX Boo and SV Peg in the 22-GHz maser line of water vapour with single-dish telescopes. The monitoring period covered two decades for RX Boo (1987-2007) and 12 years for SV Peg (1990-1995, 2000-2007). In addition, maps were obtained of RX Boo with the Very Large Array over several years. We find that most of the emission in the circumstellar shell of RX Boo is located in an incomplete ring with an inner radius of 91mas (15AU). A velocity gradient is found in a NW-SE direction. The maser region can be modelled as a shell with a thickness of 22AU, which is only partially filled. The gas crossing time is 16.5 years. The ring-like structure and the velocity gradient remained stable for at least 11 years, while the maser line profiles varied strongly. This suggests that the spatial asymmetry is not accidental, so that either the mass loss process or the maser excitation conditions in RX Boo are not spherically symmetric. The strong variability of the maser spectral features is mainly due to incoherent intensity fluctuations of maser emission spots, which have lifetimes of the order of 1 year. We found no correlation between the optical and the maser variability in either star. The variability properties of the SV Peg masers do not differ substantially from those of RX Boo. There were fewer spectral features present, and the range of variations was narrower. The maser was active on the >10-Jy level only 1990-1992 and 2006/2007. At other times the maser was either absent (<1Jy) or barely detectable. The variability of H₂O masers in the SRVs RX Boo and SV Peg is due to the emergence and disappearance of maser clouds with lifetimes of ∼1 year. The emission regions do not evenly fill the shell of RX Boo leading to asymmetry in the spatial distribution, which persists at least an order of magnitude longer.