Exploring the extended atmospheres of AGB stars

Licentiate thesis, 2024

Stars evolve along various paths depending primarily on their initial masses, undergoing changes in internal structure and processes that affect their temperature, size, and surface abundances. Low- to intermediate-mass stars ($\approx$1–8 M$_\odot$) enter the Asymptotic Giant Branch (AGB) phase in the later stages of their evolution. During this phase, these stars expand to astronomical-unit scales and experience substantial mass loss, ejecting material into the interstellar medium. This mass loss, which contributes significantly to the universe’s heavy element enrichment—including elements essential for the formation of life—originates in the stars’ extended atmospheres.

While it is understood that mass loss originates in the stellar atmosphere through outflows driven by radiative acceleration of dust, there remain many gaps in our knowledge, such as the small-scale variability of the extended atmosphere, stellar variations across periods, and the processes of dust formation and mass loss, to name a few. AGB stars, are among the largest stars in the galaxy, making them ideal candidates for direct observation and atmospheric resolution with current instruments. Over the past few decades, the study of AGB star atmospheres and their modeling has advanced significantly, with radio astronomy proving to be an effective tool for investigating these regions. However, the variability and complex structure of these stars has made these studies more challenging. The arrival of more powerful radio telescopes, alongside recent advances in theoretical models, has opened new avenues for studying the extended atmospheres of AGB stars.

This thesis first gives an overview of the current understanding of the AGB stars, especially their extended atmospheres, introduces a theoretical model for these atmospheres and presents the current and future capabilities for observing these stars. Then we introduce the synthetic imaging of the models at radio and millimeter/submillimeter wavelengths and present observable features in resolved and unresolved observations. We show comparison of temporal variations in the observable features and physical parameters in the extended atmosphere of the model AGB stars and present the role shocks in shaping the observed image of the star in these frequencies. We then investigate the capabilities of the current and future radio telescopes to observe the atmospheres of these stars in resolved and unresolved observations.

This thesis provides an overview of the current understanding of AGB stars, with a focus on their extended atmospheres and introduces a theoretical model for these atmospheres and discusses both current and future observational capabilities for studying these stars. The thesis then explores the synthetic imaging of the models at radio and millimeter/submillimeter wavelengths, highlighting observable features in both resolved and unresolved observations. We compare the temporal variations in the observable features and physical parameters of the extended atmospheres in the model AGB stars, examining the role of shocks in shaping the observed images at these frequencies. Finally, we assess the capabilities of current and future radio telescopes to observe the atmospheres of these stars in both resolved and unresolved observations.