Statistics of Hierarchical Systems

Statistics of hierarchical stellar system with 3 or more components is described by the number of components N, fractions of systems with different N, which strongly depends on the mass of primary component (14% of solar-type stars have N>2). Joint distributions of periods P, mass ratios, and eccentricites are of interest, although not accessible in practice. Crudely, a triple system can be viewed as two binaries. However, this simplistic model under-predicts triples and 2+2 quadruples and does not account for the preference of close (P<30 days) pairs to belong to hierarchies. An updated view of solar-type hierarchies within 100 pc, recently enhanced using Gaia, is presented; the 2208 known systems are still about 1/4 of their total estimated number. Distinctive families (populations) of hierarchies are tentatively identified in the multi-parameter space and illustrated by examples.