28SiO, 29SiO and 30SiO excitation: effects of infrared line overlaps

Abstract

We have investigated the influence of several line overlaps between 28SiO, 29SiO and 30SiO on maser emission from the envelopes of late-type stars using the Large Velocity Gradient radiative transfer formalism modified to account for overlaps. Our model includes: (i) 40 rotational levels for each of the v=0 to 4 vibrational states and each isotopic species; (ii) collisions with molecular and atomic hydrogen; and (iii) radiative 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 28SiO v=3, J=1-0 maser, and on the anomalously weak v=3, J=2-1 emission. We suggest that 28SiO maser emission in the v=4, J=5-4 line, the only one observed in this vibrational level, may also result from line overlaps. For 29SiO, infrared line overlaps successfully explain observed maser emission in the v=0, J=5-4 and v=1, 2, J=6-5 lines. 30SiO, v=0, J=2-1, and v=0, J=4-3 maser emissions could also result from line overlaps, and we find that several overlaps could explain the newly discovered maser line 30SiO v=1, J=1-0. Finally, we make some predictions for new 28SiO, 29SiO and 30SiO maser lines.