Circumstellar silicon chemistry and the SiO maser

We have investigated the effects of stellar chromospheric UV radiation on the composition of gaseous layers around O-rich red-giant stars losing mass, with particular attention to the almost static layers recently identified from infrared spectra. The UV radiation can strongly inhibit molecular association. Si-chemistry is studied in the steady-state approximation, and some simple results concerning time-scales, shell optical depths and photoelectric heating rates are deduced. Around α Orionis we predict that CO should be fully associated but that hydrogen should be in atomic form. We show that a chemical pump for SiO masers would operate too slowly to explain the observed photon emission rate, and propose some observational diagnostics of circumstellar molecular processes. (author)