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 model is presented. The model is based on the requirement of a vanishing luminosity perturbation and investigation of the limit of dominant radiation pressure. The run of opacity is responsible for the existence of strange modes and is found by considering a properly defined sound speed. Opacity peaks acoustically decouple different parts of the stellar envelope, each of them generating a spectrum of sonic modes, some of 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. Provided the fraction of gas pressure is small enough, instability is caused 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, is not related to the classical kappa-mechanism or any other Carnot-type process.