Sub-au imaging of water vapour clouds around four asymptotic giant branch stars

We present MERLIN maps of the 22-GHz H2O masers around four low-mass late-type stars (IK Tau, U Ori, RT Vir and U Her), made with an angular resolution of similar to15 milliarcsec and a velocity resolution of 0.1 km s(-1). The H2O masers are found in thick expanding shells with inner radii similar to6 to 16 au and outer radii four times larger. The expansion velocity increases radially through the H2O maser regions, with logarithmic velocity gradients of 0.5-0.9. IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset between the near and far sides of the shell, which suggests that the masers are distributed in oblate spheroids inclined to the line of sight. U Ori and U Her have elongated poorly filled shells with indications that the masers at the inner edge have been compressed by shocks; these stars also show OH maser flares. MERLIN resolves individual maser clouds, which have diameters of 2-4 au and filling factors of only similar to0.01 with respect to the whole H2O maser shells. The circumstellar envelope velocity structure gives additional evidence the maser clouds are density-bounded. Masing clouds can be identified over a similar time-scale to their sound crossing time (similar to2 yr) but not longer. The sizes and observed lifetimes of these clouds are an order of magnitude smaller than for those around red supergiants, similar to the ratio of low-mass:high-mass stellar masses and sizes. This suggests that cloud size is determined by stellar properties, not local physical phenomena in the wind.