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

We have investigated the influence of several line overlaps between 28 SiO, 29 SiO and 30 SiO 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 28 SiO v =3 ;J =1 ! 0 maser, and on the anomalously weakv =3 ;J =2 ! 1 emission. We suggest that 28 SiO 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 29 SiO, infrared line overlaps successfully explain observed maser emission in the v =0 ;J =5 ! 4 and v = 1;2 ;J =6 ! 5 lines. 30 SiO, 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 30 SiO v =1 ;J =1 ! 0. Finally, we make some predictions for new 28 SiO, 29 SiO and 30 SiO