2004A&A...426..131S

We present sub-milliarcsecond observations of SiO masers in the late-type stars IRC+10011 and χ Cyg. We have used the NRAO Very Long Baseline Array (VLBA) to map the 43GHz (v=1,2 J=1-0) and the 86 GHz (v=1,2 J=2-1) SiO masers. All the transitions have been imaged except the v=2 J=2-1 in IRC+10011. We report the first VLBI map of the v=1 J=2-1 ²⁸SiO maser in IRC+10011 as well as the first VLBA images of SiO masers in an S-type Mira variable, χ Cyg. In this paper we have focused on the study of the relative spatial distribution of the different observed lines. We have found that in some cases the observational results are not reproduced by the current theoretical pumping models, either radiative or collisional. In particular, for IRC+10011, the v=1 J=1-0 and J=2-1 ²⁸SiO lines have different spatial distributions and emitting region sizes, the J=2-1 emission being located in an outer region of the envelope. For χ Cyg, the distributions also differ, but the sizes of the masing regions are comparable. We suggest that the line overlaps between ro-vibrational transitions of two abundant molecular species, H₂O and ²⁸SiO, is a possible explanation for the discrepancies found between the observations and the theoretical predictions. We have introduced this overlapping process in the calculations of the excitation of the SiO molecule. We conclude that the line overlaps can strongly affect the excitation of SiO and may reproduce the unexpected observational results for the two sources studied.