Simultaneous VLBI observations of the SiO masers of the J = 1 → 0 rotational line in the v = 1 and v = 2 vibrational levels toward Mira variables are presented. Because SiO maser lines are formed deep in circumstellar envelopes they serve as a unique tool to study the innermost envelopes of evolved stars. Although the first interpretation of SiO maser emission was made in 1974, observational features are only partially explained by models which have been suggested since then. Positional coincidence of the J = 1 → 0 masers of v = 1 and v = 2 has been argued as a way to distinguish among the maser pumping models, but it requires simultaneous observations of the two lines using high resolution. We have developed the technique for such observations during the last few years and here we report successful results of our simultaneous observations of the SiO v = 1 and v = 2 J = 1 → 0 masers using the Very Long Baseline Array (VLBA). We discuss the pumping mechanism in terms of our observational results.
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