Circumstellar envelope structure of late-type stars as revealed by MERLIN observations of 22-Ghz water masers

Abstract

We present MERLIN data of 22-GHz H2O maser emission from the inner circumstellar envelopes of two Mira variables, one semiregular variable star and three red supergiant stars. The H2O masers occur at radial distances of 1013–1014 m from the central star and have generally aspherical distributions. The size of the maser region increases with mass-loss rate, as predicted by Cooke & Elitzur. Evidence is presented for radial acceleration of the outflow through the H2O maser zone. The structures of the inner circumstellar envelopes of the Mira variables IK Tau and U Her and the semiregular variable RT Vir are discussed in detail. Comparison between the MERLIN data on IK Tau and VLA data obtained 16 months earlier shows that the strongest masers have persisted over this period, but that there has been a general expansion of the H2O envelope by 6 per cent, which corresponds to proper motions of ∼ 6 mas yr–1 for individual masers.