SiO cooling instability in the envelopes of cool giant stars
In the outer atmospheres of oxygen-rich giant and supergiant stars, the formation of the silicon monoxide molecule not only represents a first step toward the production of dust but may also produce an efficient radiative cooling agent. Molecular cooling occurs as a thermal instability in many cases with an onset at 2000 to 3500 K, rapidly reducing the temperature by as much as 1000 K. This process may occur in the upper photosphere-lower chromosphere of such stars and can have profound dynamical consequences. The process occurs far from LTE, on a time scale controlled by the reaction rates for SiO formation. SiO masers may be indicative of this process. The molecule CS which may play a similar role in carbon stars and serve as the basis for carbonaceous dust production.