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P. J. Diamond (MERLIN/VLBI National Facility)
The study of circumstellar envelopes has matured considerably in the past few years. Sensitive arcsecond-scale observations of the thermal emission from numerous molecules are now possible using the current generation of mm-wave interferometers, these provide astronomers with detailed information on the large-scale structures, dynamics and the overall chemistry of the envelopes. IR studies by ISO have also provided an unprecedented view of the chemistry of many more molecules. IR and optical interferometric and speckle observations from the ground are also opening up a new area of study enabling us to begin to directly measure the pulsations of the stellar atmosphere and the location and structure of the dust emission.
Maser emission from SiO, H2O and OH are also prevalent in the envelopes of oxygen-rich stars. Although one is limited in the chemistry that can be studied through observations of masers this is compensated for by the fact that masers are superb probes of the structure, kinematics and polarization properties of the envelope. With VLBI astronomers can obtain images with spatial resolutions as small as 250 microarcseconds at 43 GHz, this corresponds to 0.075 AU. In this review I shall discuss the current state of the art of such high resolution observations of all three common masing molecules. I shall concentrate in particular on the knowledge being gained through monitoring of SiO masers, observations of different rotational and vibrational transitions of SiO and polarization observations of both OH and H2O masers.
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