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J.M. Gardner (Smith College), R.B. Phillips (MIT Haystack Observatory), D.A. Boboltz (U.S. Naval Observatory)
Over a period of six months in 1999-2000, six epochs of VLBA data at 7mm (43 GHz) were taken of J = 1 to 0, v=1 and v=2 SiO maser emission around Mira (o Ceti). Images were created from each epoch and compared to study the motion of the masing regions within Mira's stellar envelope. In each transition, the masing regions appeared in a broken, 2 to 3 AU ring about the star. Mira's sparse SiO maser shell contrasts with the contiguous rings seen from such AGB stars as R Cas and TX Cam.
The data encompass a period of time in which Mira peaks in visual wavelengths. The peak for the SiO masers lagged behind the visual peak, as expected. However, this lag was different between the v=1 and v=2 vibrational states. These images, which resemble a stop-action movie of Mira's masing regions, are the highest fidelity look into its envelope achieved to date.