Strange Cepheids driven by the kappa-mechanism

Strange modes have been a matter of vigorous debate as much because of the variety of stars in which they have been found as for their puzzling nature. This discussion concentrates on the adiabatic (and nearly-adiabatic) limit, arguing that not only does the problem become more simple, but that the simplification retains the essential properties which explain the nature of the strange modes. Based on this model, we conclude that no new mechanism is needed to explain the instability of strange modes: the $\kappa$ mechanism is still at work, although in a highly constrained configuration. On a more practical level, the adiabatic limit is interesting because the classical variable stars -- Cepheids and RR Lyrae -- are weakly non-adiabatic pulsators, and the appearance of strange modes in Cepheid models motivated this investigation (Buchler 1996).