Chaotic Oscillations in a Simple Stellar Model — A Mechanism for Irregular Variability

Abstract

We propose a mechanism which can cause stellar pulsations of an unexpectedly complex nature. The mechanism is based on the existence of multiple hydrostatic equilibria of a stellar model with a given entropy profile. If two such dynamically stable equilibria are in thermal imbalance, a situation can arise in which the star will be thermally driven from one hydrostatic equilibrium to the other and will oscillate (on a dynamical timescale). In such a case, depending on the ratio of the dynamical to the thermal timescale, the model can exhibit relaxation oscillations, chaotic variability or limit cycles. We demonstrate the mechanism by a simple model of a late-type red giant in which the extended envelope contains large overlapping ionization zones.