VLBA observations of SiO masers towards Mira variable stars

We present new total intensity and linear polarization VLBA observations of the nu=2 and nu=1 J=1-0 maser transitions of SiO at 42.8 and 43.1 GHz in a number of Mira variable stars over a substantial fraction of their pulsation periods. These observations were part of an observing program that also includes interferometric measurements at 2.2 and 3.6 micron \citep{Mennesson2002}; comparison of the results from different wavelengths allows studying the envelope independently of the poorly known distances to these stars. Nine stars were observed at from one to four epochs during 2001. The SiO emission is largely confined to rings which are smaller than the inner radius of the dust shells reported by \citet{Danchi1994}. Two stars (U Orionis, R Aquarii) have maser rings with diameters corresponding to the size of the hot molecular layer as measured at 3.6 micron; in the other cases, the SiO rings are substantially larger. Variations of ring diameter for most, but not all stars, had an rms amplitude in agreement with the models of \citet{Humphreys2002} although the expected relationship between the diameter and pulsation phase was not seen. The ring diameter in U Orionis shows remarkably small variation. A correlation between the 2.2/3.6 mum diameter ratio with that of the SiO/3.6 mum diameter ratio is likely due to differences in the opacities at 2.2 and 3.6 mum in a molecular layer. A further correlation with the inner size of the dust shell reported by \citet{Danchi1994} suggest some differences in the temperature structure. Clear evidence is seen in R Aquarii for an equatorial disk similar to that reported by \citet{Hollis2001}; rotation is possibly also detected in S Coronae Boralis.