16 - Miras and Long-Period Variables

Summary

Basic Model Ingredients

From Chapters 1 and 2 we know that low- to intermediate-mass stars (1 to 8 M⊙) are found to evolve along the Asymptotic Giant Branch (AGB). These stars are surrounded by large, extended dust shells and are characterized by pronounced time variations. This is particularly true for the main constituents of the AGB – Miras and Long-Period Variables (LPVs) – the light curves of which exhibit a more or less well-defined periodicity, in this way showing a kind of an oscillating behavior. This is assumed to be caused by pulsations of the deeper layers driven by kind of a κ mechanism (see e.g., Section 1.4.4). By these internal pulsations, hydrodynamic waves are generated that travel outward into a medium with decreasing density and temperature, causing the waves to increase in amplitude and finally grow and steepen to shock waves.

These shock fronts moving outward through the atmosphere have a significant bearing on the actual local thermal and chemical state of the shell, producing either favorable or unfavourable conditions for grain nucleation and growth, respectively. By means of these processes, a complex interplay between the internal pulsation, the dynamics of the circumstellar shell, and dust formation and growth is induced, the nonlinear treatment of which allows a reliable understanding not only of the detailed shell dynamics and its particular wind characteristics but also of its detailed spectral appearance, as illustrated in Figure 16.1, where the causal interplay of the various processes that govern the local and global dynamic shell structure is sketched.