The Formation of Binaries

Binary stars produce an array of dramatic astrophysical phenomena. They allow us to probe stellar structure, nuclear physics, and gravitational wave physics. They also produce the powerful supernovae that allow us to measure the scale of the universe. Despite their importance and ubiquity, many questions remain unanswered as to how the star formation process produces the wide array of stellar multiples that we observe. A complete model for binary formation encompasses three main components. We must know: (1) the primordial population of systems, (2) the influence of the dynamical processes that reshape this distribution as stars form and natal clusters disperse, and (3) the role of binary stellar evolution. In this article I review the most prominent theories for binary formation: turbulent core fragmentation, disk fragmentation, and competitive accretion. I argue that turbulent core fragmentation at all masses, with disk fragmentation added in at the upper end of the mass spectrum, might explain the trend towards increasing multiplicity for higher mass stars. In addition, I provide a brief overview of the observational statistics and of some of the important processes that modify the primordial distribution of stellar orbits.