VLBA observations of SiO masers: arguments in favor of radiative pumping mechanisms

We have performed VLBA observations of the SiO v=1 and v=2 J=1-0 masers in two AGE stars, TX Cam and IRC +10011. We confirm the ring-like spatial dist ribution, previously found in several AGE objects, as well as the tangentia l polarization pattern, already reported for TX Cam. Both properties, that seem to be systematic in this kind of objects, are characteristic of radiat ively pumped SiO masers. On the contrary, we do not confirm the previous re port on the spatial coincidence between the J=1-0 v=1 and 2 masers, a resul t that would have argued in favor of collisional pumping. We find that both lines sometimes arise from nearby spots, typically separated by 1-2 mas, b ut are rarely coincident. The discrepancy with previous results is explaine d by the very high spatial resolution of; our observations, similar to 0.5 mas, an order of magnitude better than in the relevant previously published experiment.