Architecture of Hierarchical Stellar Systems and their Formation
  Accumulation of new data on stellar hierarchical systems and the progress in
numerical simulations of their formation open the door to genetic
classification of these systems, where properties of a certain group (family)
of objects are tentatively related to their formation mechanisms and early
evolution. A short review of the structure and statistical trends of known
stellar hierarchies is given. Like binaries, they can be formed by the disk and
core fragmentation events happening sequentially or simultaneously and followed
by the evolution of masses and orbits driven by continuing accretion of gas and
dynamical interactions between stars. Several basic formation scenarios are
proposed and associated qualitatively with the architecture of real systems,
although quantitative predictions for these scenarios are still pending. The
general trend of increasing orbit alignment with decreasing system size points
to the critical role of the accretion-driven orbit migration, which also
explains the typically comparable masses of stars belonging to the same system.
The architecture of some hierarchies bears imprints of chaotic dynamical
interactions. Characteristic features of each family are illustrated by several
real systems.
