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Formation and Evolution of Hierarchical Systems

Published online by Cambridge University Press:  22 February 2018

Sverre J. Aarseth
Sverre J. Aarseth*
Affiliation:
Institute of Astronomy, University of Cambridge, UK Madingley Road, Cambridge CB3 0HA, UK

Abstract

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Initial distributions of primordial binaries used in realistic N-body simulations give rise to bound subsystems of different multiplicity. An examination of the formation process shows that the most compact triples arise via binary-binary collisions, whereas higher-order systems have diverse origins. Low formation rates are compensated by long life-times, leading to a significant population building up. Being fairly energetic, the outer orbit tends to shrink by further encounters. In addition, external perturbations also modify the eccentricity and may create conditions for instability. Onecharacteristic outcome is internal disruption by the sling-shot mechanism. Such interactions are often sufficiently energetic to produce high-velocityescapers and it is not uncommon for triples and quadruples to be ejected. For high inclinations, the eccentricity growth induced by the Kozai effect may lead to significant shrinkage of the inner binary orbit by tidal circularization. If some of the components were spit into a dynamically inactive ultra-hard binary, the hierarchies would be attributed an even higher multiplicity. One implication of these results is that a proportion of multiple systems observed in the field may have been formed in a dynamical environment.

Resumen

Resumen

Se utilizan distribuciones iniciales de binarias primordiales en simulaciones realistas de N-cuerpos que dan origen a subsistemas ligados de diferente multiplicidad. Un examen del proceso de formación muestra que las triples más compactas se originan en colisiones binaria-binaria, mientras que los sistemas de orden mayor tienen orígenes diversos. Las tasas de formación son bajas, pero se compensan con vidas medias largas, lo cual lleva a un aumento significativo de la población de sistemas múltiples. Al ser bastante energética, la órbita exterior tiende a contraerse en los encuentros subsecuentes. Además, las perturbaciones externas también modifican la excentricidad, y pueden crear condiciones propicias para la inestabilidad. Un resultado característico es la destrucción interna por el efecto honda. Estas interacciones son con frecuencia suficientemente energéticas como para producir escapes a alta velocidad, y no es raro que algunas dobles o triples salgan expelidas. Para inclinaciones altas, el crecimiento de la excentricidad inducido por el mecanismo de Kozai puede llevar a una contracción apreciable de la órbita de la binaria interior, a causa de la circularización por efecto de marea. Si algunas de las componentes se fisionan para formar binarias ultracerradas, dinámicamente inactivas, las jerarquías formarían multiplicidades de aún mayor orden. Estos resultados implican que una parte de las múltiples de campo observadas pudieron haberse formado en un ambiente dinámico.

Keywords

BinariesGeneral — MethodsN-Body Simulations — Open Clusters and Associations
Type
Theoretical Approaches to Multiple Stars and Their Formation
Information
International Astronomical Union Colloquium , Volume 191: The Environment and Evolution of Double and Multiple Stars , August 2004 , pp. 156 - 162
Copyright
Copyright © Instituto de Astronomia – Mexico 2004

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