1995A&A...302..797D

Result. are presented of an extensive new model of circumstellar SiO masers, involving 200 rovibrational levels for v=0 to 4, including saturated maser radiation transport. The model incorporates radiative and collisional pumping, involving both dust and stellar radiation. Unsaturated gain coefficients are calculated for conditions appropriate to both M-Miras and supergiants, grouped together, and for OH/IR stars. We find that the characteristics of M-Miras and supergiants, displaying many maser lines simultaneously, are reproduced through collisional pumping at 1500K, typically ∼1 stellar radius from the stellar surface, at a number density of ∼5x10⁹H₂/cm3, in an environment experiencing only a weak dust continuum radiation field. Good agreement is established between the physical conditions calculated in theoretical models of M-Mira atmospheres and the parameters required by our model to yield the observed SiO masers. SiO masers in OH/IR stars form under distinctly different conditions, through radiative pumping in an environment experiencing a powerful dust radiation field, at typically an order of magnitude higher number density, with a variety of physical conditions reflecting the diverse character of these objects. For both M-Miras and supergiants and for OH/IR stars, certain line ratios are correctly reproduced by the unsaturated gain coefficients, and other line ratios are not. For example, the unsaturated gain coefficient in v=2, J=1-0 is always too weak in our calculations. Saturated SiO maser propagation is also considered, using a treatment which includes coupling of the saturating effects of the masers to the kinetic scheme involved in the inversion pumping (Field & Gray 1988). Competitive gain, arising through saturation, causes qualitatively new behaviour to arise in low J transitions. An important consequence of maser saturation in M-Miras and supergiants is that the v=2, J=1-0 transition becomes the dominant maser in v=2, as observed. Unsaturated population inversions may therefore be a poor guide to the saturated maser intensities in M-Miras and supergiants.
Result. for OH/IR stars however suggest that these stars may support only weakly saturated SiO masers, without significant competitive gain on maser propagation. There is a striking correspondence between results reported here and VLBI data reported in Miyoshi et al. 1994, published shortly after this work was submitted for publication.