Despite significant advances in recent decades in our understanding of the evolution and internal structure of stars, many aspects of theory remain untested or have been shown to disagree with observations. Evolutionary models are widely used by astronomers to infer various properties of stars such as their masses, ages, or metallicities, but our confidence in these models is limited to some extent by those disagreements and by a lack of observational constraints. The validation of theory is best made through a detailed comparison with accurate measurements of quantities such as the stellar mass, the radius, the effective temperatures, or the luminosities, of stars in carefully selected binary systems of known chemical composition. However these determinations are not trivial to make with the required precision. In this project, high quality spectroscopic, photometric, or astrometric observations are being obtained and analyzed to make the determinations of the absolute stellar properties with relative errors, in the case of the masses and radii, at the current state-of-the-art, and to perform "critical tests" of stellar evolution theory. More than that, the goal is to provide so many observational constraints for a given binary system that only the most realistic sets of models (if any) may satisfy all the observations within the errors. Key binary systems are being identified that provide the best tests of the models, with particular emphasis on low-mass stars, evolved stars and very young (pre-main-sequence) stars for which virtually no constraints are available, as well as on stars with chemical compositions very different from the Sun (metal-poor).
Broader Impact. This project is advancing our understanding of several poorly known areas of stellar astrophysics such as convection (mixing length theory, overshooting), internal structure, stellar rotation, mass loss, and tidal evolution, allowing much more reliable determinations of parameters such as the ages of stars. These stellar parameters have profound cosmological implications and provide a more solid framework for the study of the origin and evolution of substellar objects such as brown dwarfs and extrasolar planets, subjects of great interest for the public and scientists alike. The dissemination of the results to the scientific community, mainly in the form of peer-reviewed publications, is an integral part of this work. It provides excellent opportunities for training by integrating front-line research into the experience of learning, allowing students to actively participate in all aspects of the study of crucial binary systems.
