Formation of Binary Systems

Abstract

AbstractRecent observational studies of the properties of binary systems among young stars indicate that the majority of binaries are formed very early in the history of a star, perhaps during the protostellar collapse. Observational studies of these early phases also point to the fact that most stars are formed in binary or multiple systems. Major observational facts to be explained include the present-day overall binary frequency and how it varies with primary mass,1 the non-negligible occurrence of multiple systems, and the distributions of period, eccentricity, and mass ratio among the individual binaries. Theoretical calculations of the collapse of rotating protostars during the isothermal phase indicate instability to fragmentation into multiple systems. This process in general produces systems with periods greater than a few 100 yr, although somewhat shorter periods are possible. Fragmentation during later, optically thick, phases of collapse tends to be suppressed by pressure effects. Therefore, major theoretical problems remain concerning the origin of close binaries. Fission of rapidly rotating stars, tidal capture, and three-body capture have been shown to be improbable mechanisms for formation of close binaries. Mechanisms currently under study include gravitational instabilities in disks, orbital interactions and disk-induced captures in fragmented multiple systems, hierarchical fragmentation, and orbital decay of long-period systems. Single stars, on the other hand, most likely result from escape from multiple systems during the early phases of evolution, but they could also form by the collapse of clouds of low angular momentum, coupled with angular momentum transport after disk formation.KeywordsAngular MomentumTriple SystemClose BinaryTauri StarCircumstellar DiskThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.