ON THE ORIGIN OF STRANGE MODES AND THE MECHANISM OF RELATED INSTABILITIES

An attempt to explain the origin of strange modes and the mechanism of related instabilities in terms of a simplified intuitive physical mod el is presented. The model is based on the requirement of a vanishing luminosity perturbation and investigation of the limit of dominant rad iation pressure. The run of opacity is responsible for the existence o f strange modes and is found by considering a properly defined sound s peed. Opacity peaks acoustically decouple different parts of the stell ar envelope, each of them generating a spectrum of sonic modes, some o f which were addressed as strange modes in previous investigations. Re -examining the classical work integral, instabilities are interpreted in terms of a phase shift between pressure and density perturbation. P rovided the fraction of gas pressure is small enough, instability is c aused for a finite range of wavenumbers by the differential character of the diffusion equation for energy transport. It is emphasized that the instability mechanism, being essentially independent of opacity, i s not related to the classical kappa-mechanism or any other Carnot-typ e process.