Relative orientation of orbits in triple stars

Statistical analysis of the relative alignment of inner and outer orbits in triple systems resulting from a dynamical decay of small-N clusters (N ≤ 10) is presented and compared to the statistics of real multiple stars. The distribution of the relative angle Φ 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 〈Φ〉 between 67◦ and 79◦. 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 Φ should be extended as it will allow one to sensitively constrain properties of initial clusters.