Arief Ahmad: Dynamical Processes in Red Giants: Pulsations, Convection, and Mass Loss of Cool, Luminous, Evolved Stars in 3D Models

Abstract

Stars on the asymptotic giant branch (AGB) mark a critical, late phase of stellar evolution. They develop extended, cool atmospheres where pulsation-driven shocks lift gas, facilitating dust condensation and acceleration of stellar winds via radiation pressure. With high mass loss rates, these stars significantly enrich the interstellar medium. Observations reveal complex, time-variable structures in their atmospheres and envelopes, shaped by dynamic variability and feedback processes.

To investigate these processes, the research described in this thesis used global 3D radiation-hydrodynamics models computed with CO5BOLD. The simulations capture the entire star, revealing interior dynamics driven by global convection, surface granulation, stochastic waves, and pulsations. Instead of imposing pulsation behaviour, the models let pulsations arise naturally from internal dynamics, enabling investigations from first-principles.

A primary result of this work is the detailed characterisation of self-excited pulsations. Analysing two model grids spanning various stellar masses, luminosities, and densities, clear relationships emerged between pulsation properties and global stellar parameters. Dominant periods obey well-known scalings, including the period-mean density relation and period-luminosity sequences seen in Miras and semiregular variables. In high-luminosity, low-density models, pulsations in the fundamental radial mode dominate with large amplitudes, while higher-density models show multiple radial overtones and low-degree non-radial modes with mode-switching behaviour over time. 

Both convection and pulsations naturally arise in the 3D models, yet their interaction is complex. We examined this interplay to understand its non-linear effects on the stellar atmosphere. The coupling influences atmospheric structure, variability amplitude, and the lifting and cooling of material, ultimately leading to dust-driven winds. The formation of inhomogeneous, dusty clumps in the outer atmosphere is an essential process contributing to the highly structured and time-dependent nature of AGB winds. Preliminary analysis presented in this thesis lays groundwork for future efforts aimed at parameterising wind properties from a 3D perspective.

The 3D models are essential to capture the dynamic behaviour of evolved stars. They naturally produce pulsations and their interaction with convection and winds, explaining observations from atmospheric variability to dusty clump formation driving mass loss. Understanding these processes is vital for advancing our knowledge of late-stage stellar evolution and the chemical evolution of galaxies.