THE CO SIO RADIATIVE INSTABILITY IN COOL STAR ATMOSPHERES REVISITED/

We revisit the formation of radiative instabilities in cool star atmos pheres and compare our results with those given by Muchmore, Nuth, & S tencel. We have considered the combined influence of CO and SiO molecu les and have computed models for a grid of effective temperatures and geometrical dilution factors for the stellar radiation. Our results ar e based on the analysis of the energy balance of gas elements with pre scribed thermodynamic properties. Our results show that radiative inst abilities are most likely primarily caused by CO, whereas SiO is expec ted to play only a minor role, except when the CO density is reduced c ompared to LTE values or the CO band can be assumed to be optically th ick. The onset of radiative instabilities is expected to be strongly m odified when dynamic phenomena such as stochastic shocks are present. Our results provide strong evidence that dust formation can most likel y not occur via a radiative instability alone. Therefore, we present a revised version of the Muchmore et al. dust formation paradigm, which also considers hydrodynamic cooling. The new paradigm is particularly relevant in cases where dust is formed relatively close to the stella r photosphere.