1998A&A...335..654B

We have observed with the IRAM interferometer at two different epochs and simultaneously the two transitions v=0, J=2 ->1 and v=1, J=2 ->1 of ²⁹SiO and ²⁸SiO in Orion IRc2. We have made the first maps of ²⁹SiO v=0, J=2 ->1 emission from Orion. These maps and properties of the ²⁹SiO spectra attest to maser emission. Our ²⁸SiO maps show the stable ring of maser spots observed in previous works. Combining our own data with published works we derive that the relative motion between the two ridges of the ²⁸SiO emission ring is less than about 0.7AU/yr over a period of 7 years. On the other hand, the weak high velocity maser features observed around 30km/s seem to move with respect to the stable ring of ²⁸SiO main emission. Our relative ²⁹SiO (v=0) and ²⁸SiO (v=1) spot maps show that most ²⁹SiO and ²⁸SiO emission features are closely related but have not the same spatial extent. We conclude that these masers are not excited in the same gas layers in agreement with pumping models which predict that various v state masers peak in different spatial regions. In addition, our maps of v=0 and v=1 emission suggest that local line overlaps due to turbulence and high gas temperature do not play a dominant role in the excitation of ²⁸SiO and ²⁹SiO , although excitation effects resulting from the overlap of Doppler-shifted ro-vibrational lines may still be significant.