Optical pumping of circumstellar SiO masers

We investigate the influence of the photospheric line spectrum on the optical pumping of circumstellar SiO molecules. As pump radiation we consider in particular the synthetic line spectrum of an M-star atmosphere (T-eff = 3000 K). The pump intensity in a given line depends sensitively on the velocity difference between envelope and photosphere. The pump intensities in different lines may vary due to the deep absorption features of the spectrum. Thus, the question whether there arises population inversion in circumstellar SiO molecules, not only depends on the SiO abundance, the density, and the velocity gradient in the envelope, but also on the absolute value of the velocity difference between envelope and photosphere. The aim of the present investigation is to find out under which physical conditions inversion occurs. We have shown that optical pumping causes maser emission without the need of collisions. Our findings differ substantially from the results of similar calculations assuming the spectrum of the pump radiation to correspond to that of a black body. We have done the calculations for three different isotopomeres of SiO and we have shown that the regions of inversion in parameter space are distinct for the various isotopomeres due to the frequency differences in the pump lines and due to the velocity difference Delta(vr) between the photosphere and the emitting volume element.