In this PhD thesis, Matthias Fabry takes a deep dive into the detailed structure and evolution of the most massive and tightest binary stars. Massive stars are predominantly born in binary systems, and recent modeling shows that about 40% of them will form a so-called contact binary during their life. In this configuration, both stars share their outer layers, forming peanut-shaped objects. Previously, evolutionary models of such systems used simplified physics and approximations, and were failing to reproduce observed properties of massive contact binaries. In his PhD work, Matthias incorporated tidal deformation and energy transfer into a modern stellar-structure and evolution framework. This culminated in the creation of extensive grids of models spanning all massive stars that would form long-lived contact binaries. In his thesis, Matthias further offers a broad introduction into the history of (computational) stellar evolution, binary stars, and the connection of massive binaries with gravitational-wave astrophysics. Not only do the methods developed here constitute a significant step forward in the detailed modeling of massive contact binaries, they are applicable to many other kinds of binary stars.

Nominated for the Springer Theses Prize by KU Leuven.

Matthias Fabry is a stellar astrophysicist currently living in suburban Philadelphia. He grew up in Tervuren, Belgium, and went to university in nearby Leuven, attending the KU Leuven for his degrees in physics and astrophysics. He obtained his PhD at the Institute of Astronomy in Leuven in May 2024, after extensively studying the structure and evolution of contact binaries, in particular those containing massive stars. During his PhD research, he published three peer-reviewed papers on the topic, and helped reinvigorate interest in this niche, which has been underdeveloped in the past decades. Currently, he is a post-doctoral scholar at the Department of Astrophysics and Planetary Science at Villanova University near Philadelphia in the USA. He is continuing to work on the problem of contact binaries (of any mass), by combining light-curve analysis with evolutionary models to determine their properties, their interaction physics, and ultimate fate. In his pastime, he likes playing golf, watching sports (especially the American ones), learn about aero- and astronautics, and play board games.