Unlike the evolution of single stars, which is to a first approximation a function only of initial mass and chemical composition, the evolution of interacting binary stars involves at minimum four parameters: the masses of each of the two stars, their orbital separation (or period), and chemical composition. The large number of parameters makes it prohibitively expensive to explore every combination possible. Furthermore, theory is not yet able to rigorously quantify many critical physical processes, most especially the extent of mass and angular momentum loss from a binary undergoing mass exchange between components. Statistical models which attempt to reproduce the distributions of observable properties of different kinds of close binary systems (population synthesis models) offer the opportunity for insight to the magnitude of these effects. These models rely upon simple parameterizations of single-star evolution models, combined with a broad, but very incomplete, understanding of how interacting binary stars evolve based on the behavior of the relatively small number of detailed binary evolutionary models in the published literature. The research program here is providing a rigorous general framework for population synthesis models, as well as exploring specific evolutionary problems of widespread interest to researchers in binary star evolution. The work encompasses an array of projects critical to progress in binary evolution theory. Construction of adiabatic and thermal-equilibrium mass loss sequences for stars spanning the entire range of the masses and evolutionary states will define mass transfer time scales appropriate to all classes of interacting binary stars. The evolution of binary stars in physical contact with each other is explored with a simultaneous binary evolution code, magnetic field coupling to binary evolution among the magnetic cataclysmic binaries, and the modeling of wind-fed magnetic cataclysmic variables.
Broader Impacts: This work includes a graduate student, now engaged in preparatory work for her Ph.D. dissertation, who will also participate in national and international conferences. Three of the major components of this research involve international collaborations, with researchers in China, Australia, and Germany. The numerical results of the stability study are to be made available in digital form, since it has broad application for all kinds of binary evolution studies. And finally, the proposed studies of donor star variability in cataclysmic variables provide the means to test for long-term stochastic variability among lower-main-sequence stars. This project also maintains and develops binary star databases.
