High-precision stellar astrophysics: testing models of stellar structure using asteroseismic, astrometric, and spectroscopic constraints

Stellar evolutionary models are crucial for much of astrophysical research, including nucleosynthesis, exoplanetary systems, and Galaxy formation. However, their internal structures have only recently been tested through asteroseismic observations, enabling precise analysis of stellar interiors. This PhD thesis explores the internal structure and evolution of low-mass core helium burning (CHeB) stars using advanced asteroseismic and Bayesian methods, aiming to refine our understanding and evaluate non-standard models that challenge traditional paradigms. The research begins with the identification of eleven red giant stars in the $Kepler$ field, revealing complex oscillation spectra indicative of very low-mass CHeB stars. These stars feature a helium core of ~0.5 $M_\odot$ and a lighter hydrogen-rich envelope, resulting in a higher coupling factor between internal pressure and gravity mode cavities compared to red clump stars. This work demonstrates the potential of asteroseismology to identify low-mass CHeB stars, advancing our understanding of stellar evolution and mass loss. The thesis also investigates the formation of such stars, focusing on the anomalously low-mass CHeB star KIC4937011 in the open cluster NGC 6819. This Li-rich star, about 1 $M_\odot$ less massive than others at the same evolutionary stage, offers a unique opportunity to explore its formation pathway. Using Bayesian techniques and binary stellar population models, this Thesis suggests that KIC4937011 formed from a common-envelope phase where the companion star did not survive, leading to significant mass loss. This finding provides insights into the evolution of stars like subdwarf B stars and metal-rich RR Lyrae. Additionally, the research examines asteroseismic signatures of structural variations near the convective core of low-mass CHeB stars, revealing how these glitches influence oscillation spectra by employing semi-analytical models. Overall, this work significantly advances our understanding of low-mass CHeB stars and their evolutionary histories, contributing to broader studies of stellar evolution and the Milky Way’s formation.