Explanation of 29 SiO , 30 SiO and high - v 28 SiO maser emission

We have developed a non–local radiative transfer code to study the effects of line overlaps between different molecular species on the emergent line intensities from interstellar and circumstellar clouds. The numerical method solves the transfer of radiation and the statistical equilibrium populations for the different molecules simultaneously, and allows a direct comparison between the intensities calculated when overlaps are included/ignored in the models. We have applied the code to explain the maser emission in the rotational lines of 29SiO, 30SiO and high-v (v ≥ 3) 28SiO observed toward oxygen–rich circumstellar envelopes. The model calculations show that the pumping mechanism for most of these lines is dominated by infrared line overlaps between 28SiO, 29SiO and 30SiO.