Multi - transition VLBA observations of circumstellar SiO

On this paper, we present simultaneous VLBA observations of the J=1–0 and J=2–1 rotational lines in the v=1 and v=2 vibra-tionally excited states of SiO (at λ ∼ 7 and 3 mm). We have mapped these four maser lines in the circumstellar envelopes of three AGB stars: IRC +10011, χ Cyg and TX Cam. We study the relative spatial distribution between these maser lines. In particular, for the two v=1 transitions , we found that the J=1–0 and J=2–1 maser spot distributions are unalike, challenging all the current theoretical pumping mechanisms for SiO masers (which predict quite similar distribution for maser lines in the same vibrational excited state).