(SiO)-Si-28, (SiO)-Si-29 and (SiO)-Si-30 excitation: effects of infrared line overlaps

We have investigated the influence of several line overlaps between (SiO)-S i-28, (SiO)-Si-29 and (SiO)-Si-30 on maser emission from the envelopes of l ate-type stars using the Large Velocity Gradient radiative transfer formali sm modified to account for overlaps. Our model includes: (i) 40 rotational levels for each of the upsilon = 0 to 4 vibrational states and each isotopi c species; (ii) collisions with molecular and atomic hydrogen; and (iii) ra diative pumping by a shell of dust grains surrounding the SiO masing layers . We have shown that several line overlaps may play a role on the excitation of the (SiO)-Si-28 upsilon = 3, J = 1 --> 0 maser, and on the anomalously w eak upsilon = 3, J = 2 --> 1 emission. We suggest that (SiO)-Si-28 maser em ission in the upsilon = 4, J = 5 --> 4 line, the only one observed in this vibrational level, may also result from line overlaps. For (SiO)-Si-29, inf rared line overlaps successfully explain observed maser emission in the ups ilon = 0, J = 5 --> 4 and upsilon = 1.2, J = 6 --> 5 lines. (SiO)-Si-30, up silon = 0, J = 2 --> 1, and upsilon = 0, J = 4 --> 3 maser emissions could also result from line overlaps, and we find that several overlaps could exp lain the newly discovered maser line (SiO)-Si-30 upsilon = 1, J = 1 --> 0. Finally, we make some predictions for new (SiO)-Si-28, (SiO)-Si-29 and (SiO )-Si-30 maser lines.