Unstable and Stable Mass Transfer in Stellar Binaries: From Common Envelopes to Circumbinary Outflows

Close binary star systems undergo dramatic mass transfer events that reshape their orbital and physical properties, ultimately producing gravitational wave sources, thermonuclear transients, and other high-energy phenomena. This thesis investigates the physics governing close binary formation and evolution, spanning both stable and unstable mass transfer, the strong tidal interactions that follow, and the transients to which mass transfer can give rise. The earlier chapters constrain the evolutionary history of close WD binaries. In Chapter II, I model a sample of double WD binaries to infer the efficiency of common envelope (CE) ejection, finding a low and nearly constant efficiency can explain their formation. These constraints inform binary population synthesis models, where uncertainties in the CE propagate to many phenomena. In Chapter III, I compute the tidal heating of ultrashort-period WD binaries self-consistently within binary evolution models. Although tidal heating enhances surface temperatures at short orbital periods, it is not the dominant luminosity source in observed systems, which are instead consistent with intrinsically young, hot WDs also formed with a low CE efficiency. In Chapter IV, I analyze the host galaxies and delay time distributions of calcium-rich gap transients versus other classes of supernova, using ZTF Census of the Local Universe data. These transients favor massive, quiescent hosts similar to those of 91bg-like supernovae, suggesting long delay times and old progenitor systems. The delay time distributions and inferred offsets from host light are consistent with a compact binary progenitor in which a prior episode of mass transfer has played a key role. The later chapters investigate stable mass transfer through hydrodynamical simulations. In Chapter V, I present three-dimensional simulations of rapid, stable mass transfer and find that gas escaping through the outer Lagrange points forms an equatorially concentrated circumbinary outflow, carrying specific angular momentum close to that of the L2 point. This circumstellar material can efficiently shrink the binary’s orbit and can interact with ejecta from a subsequent supernova, producing observable transient signatures. In Chapter VI, I extend the simulations of Chapter V, incorporating radiative cooling and simulating a wider range of mass transfer rates.