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Formation of the widest binary stars from dynamical unfolding of triple systems


The formation of very wide binary systems1,2,3, such as the α Centauri system with Proxima (also known as α Centauri C) separated from α Centauri (which itself is a close binary A/B) by 15,000 astronomical units4 (1 au is the distance from Earth to the Sun), challenges current theories of star formation, because their separation can exceed the typical size of a collapsing cloud core. Various hypotheses have been proposed to overcome this problem, including the suggestion that ultrawide binaries result from the dissolution of a star cluster—when a cluster star gravitationally captures another, distant, cluster star5,6,7. Recent observations have shown that very wide binaries are frequently members of triple systems8,9 and that close binaries often have a distant third companion10,11,12. Here we report N-body simulations of the dynamical evolution of newborn triple systems still embedded in their nascent cloud cores that match observations of very wide systems13,14,15. We find that although the triple systems are born very compact—and therefore initially are more protected against disruption by passing stars16,17—they can develop extreme hierarchical architectures on timescales of millions of years as one component is dynamically scattered into a very distant orbit. The energy of ejection comes from shrinking the orbits of the other two stars, often making them look from a distance like a single star. Such loosely bound triple systems will therefore appear to be very wide binaries.

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Figure 1: Semimajor axes of stable and unstable bound triple systems.
Figure 2: Statistical properties of stable, unstable, and disrupted triple systems.
Figure 3: Stable and unstable hierarchical triple systems at 1 Myr and at 100 Myr, and the maximum extent a (1 +  e ) of a triple system as a function of time.
Figure 4: Frequency of stable, unstable and disrupted triple systems as a function of projected separation.

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We thank C. J. Clarke, M. B. N. Kouwenhoven and A. Tokovinin for comments. B.R. thanks the European Southern Observatory and Tuorla Observatory for hospitality during the period when this paper was written, and H.-F. Chiang and C. Aspin for providing additional computer facilities. This work was supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement number NNA09DA77A issued through the Office of Space Science. This research has made use of the SIMBAD database, operated at Centre de Données astronomiques de Strasbourg, Strasbourg, France, and of NASA’s Astrophysics Data System Bibliographic Services.

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B.R. conceived the idea, carried out the simulations and data analysis, and wrote the paper. S.M. developed the code and wrote the software tools for analysis.

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Correspondence to Bo Reipurth.

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The authors declare no competing financial interests.

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Reipurth, B., Mikkola, S. Formation of the widest binary stars from dynamical unfolding of triple systems. Nature 492, 221–224 (2012).

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