The spatial and velocity structure of circumstellar water masers

We report observations with the VLA of 22 GHz H2O maser emission in the cir cumstellar envelopes of 11 late-type stars. We model the maser emission tow ards six of them (IRC +60169, R Crt, U Her, RX Boo, R LMi, and R Gas) with a computer program we have developed, that fits three-dimensional Gaussian sources (in position and velocity) to the measured intensity distribution. This approach to the problem is superior to the standard procedure of fitti ng two-dimensional Gaussian models to images for each frequency channel, an d it is shown to be a powerful method of identifying blended and/or weak ma ser features. As a result, we find that the H2O maser emission is located in a thin shell expanding from the star in four cases, despite of the fact that the emissi on is clumpy and the shells appear incomplete. Moreover, our maps indicate that the H2O maser emission comes from inner parts of the circumstellar env elope, which are comparable in extent to the regions in which dust grains f orm and in which the expanding envelope has not reached its terminal veloci ty. Our analysis also provides the spectral linewidth of each maser feature, wh ich contains information about the physical conditions in the emitting regi on. In particular, it provides a tool to discriminate whether or not these masers are saturated as, for unsaturated masers, theory predicts that a cor relation between the strongest features and the narrowest linewidths should exist. This behaviour is found in at least U Her, RX Boo, and R Cas.