The spatial and velocity structure of circumstellar water masers

We report observations with the VLA of 22 GHz H2O maser emission in the circumstellar envelopes of 11 late-type stars. We model the maser emission towards six of them (IRC +60169, R Crt, U Her, RX Boo, R LMi, and R Cas) 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 fitting two-dimensional Gaussian models to images for each frequency channel, and it is shown to be a powerful method of identifying blended and/or weak maser features. As a result, we find that the H 2O maser emission is located in a thin shell expanding from the star in four cases, despite of the fact that the emission is clumpy and the shells appear incom- plete. Moreover, our maps indicate that the H2O maser emission comes from inner parts of the circumstellar envelope, which are comparable in extent to the regions in which dust grains form and in which the expanding envelope has not reached its termi- nal velocity. Our analysis also provides the spectral linewidth of each maser feature, which contains information about the physical conditions in the emitting region. In particular, it provides a tool to discriminate whether or not these masers are saturated as, for unsaturated masers, theory predicts that a correlation between the strongest features and the narrowest linewidths should exist. This behaviour is found in at least U Her, RX Boo, and R Cas.