Studies of circumstellar shells in AGB stars by mm-VLBI observations of maser emission
VLBI observations of maser emission are a basic tool to study the circumstellar envelopes (CSEs) around evolved stars, mainly around AGB and post-AGB stars. The maser lines of water and silicon monoxide are particularly intense. They provide us with high spatial resolution data on the very inner CSEs around AGB stars, including the pulsating layers previous to grain formation and outer regions where the fast expansion characteristic of such envelopes is already present. The analysis of the pumping mechanism of SiO masers and of the physical conditions in the emitting clumps requires accurate maps of the various lines, which show different excitation requirements. We will discuss recent observations of SiO masers with the Global Millimeter VLBI Array (GMVA) which, when compared with the maps of other SiO lines, provide a new view into the physics of these AGB envelopes.