Astronomy & Astrophysics

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Issue		A&A Volume 384, Number 3, March IV 2002
Page(s)		1030 - 1037
Section		Formation, structure and evolution of stars
DOI		10.1051/0004-6361:20020105

A&A 384, 1030-1037 (2002)
DOI: 10.1051/0004-6361:20020105

Relative orientation of orbits in triple stars

M. F. Sterzik¹ and A. A. Tokovinin²

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
² Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile

(Received 6 December 2001 / Accepted 15 January 2002)

Abstract
Statistical analysis of the relative alignment of inner and outer orbits in triple systems resulting from a dynamical decay of small- N clusters ( $N \le10$ ) is presented and compared to the statistics of real multiple stars. The distribution of the relative angle $\Phi$ between the angular momentum vectors of inner and outer orbits in triple stars formed by decay is shown to depend on the initial cluster conditions like geometry, mass function, rotational and thermal energy. For a realistic set of initial conditions, a modest alignment of orbital momentum vectors is found, in good agreement with the latest observational data on visual multiple stars exhibiting an average $\langle \Phi \rangle$ between $67^\circ$ and $79^\circ$ . The relation between eccentricities of outer orbits and period ratios for both simulated and real triples is consistent with a slightly adjusted formulation of the stability criterium by Mardling & Aarseth (2001). Dynamical decay can therefore explain the weak correlation of orbital orientations observed in multiple stars. Using modern high-resolution techniques, the observed statistics of $\Phi$ should be extended as it will allow one to sensitively constrain properties of initial clusters.

Key words: stars: binaries: general -- visual -- stars: formation -- stellar dynamics

Offprint request: M. Sterzik, msterzik@eso.org

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