VLBA SiO observations of bright O-rich AGB stars

We present the results of observations of the SiO masers at 7 mm wavelength in several oxygen rich Asymptotic Giant Branch (AGB) stars. These observations were coordinated with observations using the ESO VLTI/ MIDI interferometer at 10 mu m wavelength. These stars are o Ceti (Mira), U Herculis (U Her), OH26.5+0.6, R Leonis (R Leo), and W Hydrae (W Hya). Both total intensity and linear polarizations are imaged. Linear radial features seen in o Ceti may be associated with an X-ray flare seen a year earlier by Karovska et al. (2005). Similar linear features are also seen in R Leo, U Her and OH26.5+0.6. These features typically show systematic radial velocity variations and a tendency for the linear polarization to be perpendicular to the direction of elongation, suggesting that magnetic fields are dynamically important. A wide radial extent of the maser rings observed in U Herculis and OH26.5+0.6 is interpreted as meaning that the outer boundaries of the masing regions in these stars are not truncated by the condensation of SiO onto dust grains.