MULTIPLICITY IN EARLY STELLAR EVOLUTION
B. Reipurth (University of Hawaii, Institute for Astronomy, Hilo, United States),
A.P. Boss (Carnegie Institution, Department of Terrestrial Magnetism, United States),
C.J. Clarke (University of Cambridge, Institute of Astronomy, United Kingdom),
S.P. Goodwin (University of Sheffield, Department of Physics and Astronomy, United Kingdom),
L.F. Rodriguez (Universidad Nacional Autonoma de Mexico, Mexico),
K.G. Stassun (Vanderbilt University, Department of Physics and Astronomy, United States),
A. Tokovinin (Cerro Tololo Inter-American Observatory, Chile),
H. Zinnecker (NASA Ames Research Center, SOFIA Science Center, Germany)
Observations from optical to centimeter wavelengths have demonstrated that multiple systems of two
or more bodies is the norm at all stellar evolutionary stages. Multiple systems are widely agreed to
result from the collapse and fragmentation of cloud cores, despite the inhibiting influence of magnetic
fields. Surveys of Class 0 protostars with mm interferometers have revealed a very high multiplicity
frequency of about 2/3, even though there are observational difficulties in resolving close protobinaries,
thus supporting the possibility that all stars could be born in multiple systems. Near-infrared adaptive
optics observations of Class I protostars show a lower binary frequency relative to the Class 0 phase,
a declining trend that continues through the Class II/III stages to the field population. This loss of companions
is a natural consequence of dynamical interplay in small multiple systems, leading to ejection
of members. We discuss observational consequences of this dynamical evolution, and its influence on
circumstellar disks, and we review the evolution of circumbinary disks and their role in defining binary
mass ratios. Special attention is paid to eclipsing PMS binaries, which allow for observational tests of
evolutionary models of early stellar evolution. Many stars are born in clusters and small groups, and we
discuss how interactions in dense stellar environments can significantly alter the distribution of binary
separations through dissolution of wider binaries. The binaries and multiples we find in the field are the
survivors of these various destructive processes, and we provide a detailed overview of the multiplicity
statistics of the field, which form a boundary condition for all models of binary evolution. Finally we
discuss various formation mechanisms for massive binaries, and the origin of massive trapezia and
their role in the dynamical evolution of clusters.
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