Optical pumping of circumstellar SiO masers

We investigate the influence of the photospheric line spectrum on the optical pumping of circumstellar CO. As pump radiation we consider in particular the photospheric spectrum of a cool carbon star (T eff =2600 K). This spectrum shows deep absorption lines in the wavelength range of the pump lines (4.6 μ). Of particular importance for the pump process is the velocity difference between the photosphere and the envelope.