Unveiling the Interaction Physics of Giant Binary Stars

The overarching scientific goal is to significantly advance our understanding of how giant stars interact with (often hidden) companions in binary systems—an area central to stellar and exoplanetary evolution. The work will unfold across four interconnected strands: (S1) In collaboration with expert observational astronomers in Sweden and the UK, we will compile a homogeneous dataset of UV, optical, infrared, and (sub)millimetre observations of evolved giant stars showing indirect evidence of binary interaction. Using the Leuven-based radiative transfer code MAGRITTE and retrieval tool POMME, we will infer key physical parameters such as mass-loss rates, thermodynamic and chemical properties. (S2) With experts on Gaia astrometry and interferometry from ESO (Santiago), we will develop a novel computational framework that combines (sub)millimetre interferometric data with optical astrometry from Hipparcos and Gaia DR3/DR4, enabling the derivation of full orbital solutions for giant binary systems. (S3) Together with statistical experts in Cambridge and Leuven, we will ensure robust propagation of observational and theoretical uncertainties throughout the data-to-model pipeline. (S4) In collaboration with theoretical astrophysicists from Cambridge and Leuven, we will explore both starplanet and star-star interactions using high-resolution hydrodynamic simulations. We will incorporate detailed physics of wind–accretion flows, tidal forces, and angular momentum exchange—pushing beyond current stateof-the-art models. This work will provide predictive models that can guide and interpret high-precision datasets. This integrated approach—spanning observations, modelling, and statistics—will yield new insights into the complex physics of interacting giant binaries. The timing is critical. In 2025, we detected the first close-in companion to a giant star using ALMA astrometry (Nature Astronomy), guiding target selection for new peer-reviewed Large Programs at international observatories. The upcoming Gaia DR4 release (end 2026) will offer complementary optical data. Combined, these datasets will unlock full orbital solutions and new constraints on mass transfer, tidal dissipation, and eccentricity pumping. The sabbatical period offers a unique opportunity to fully commit to this research at a moment of exceptional scientific momentum.