Multi-transition VLBA observations of circumstellar SiO masers
  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 vibrationally excited states of SiO (at 7 and 3 mm wavelengths). We have mapped these four maser lines in the circumstellar envelopes of three AGB stars: IRC +10011, Chi 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). 
R. Soria-Ruiz (1), F. Colomer (1), J. Alcolea (1), J.F. Desmurs (1),   V. Bujarrabal (1), K.B. Marvel (2), P.J. Diamond (3),...