We report on a high-spatial-resolution survey for binary stars in the periphery of theOrion Nebula Cluster, at 5-15 arcmin (0.65 – 2 pc) from the cluster center. We observed 228 starswith adaptive optics systems, in order to find companions at separations of 0.13 – 1.12 arcsec(60 – 500 AU), and detected 13 new binaries. Combined with the results of Petr (1998), wehave a sample of 275 objects, about half of which have masses from the literature and highprobabilities to be cluster members. We used an improved method to derive the completenesslimits of the observations, which takes into account the elongated point spread function of starsat relatively large distances from the adaptive optics guide star. The multiplicity of stars withmasses >2 M_⊙is found to be significantly larger than that of low-mass stars. The companionstar frequency of low-mass stars is comparable to that of main-sequence M-dwarfs, less thanhalf that of solar-type main-sequence stars, and 3.5 to 5 times lower than in the Taurus-Aurigaand Scorpius-Centaurus star-forming regions. We find the binary frequency of low-mass starsin the periphery of the cluster to be the same or only slightly higher than for stars in thecluster core (< 3' from 01C Ori). This is in contrast to the prediction of the theory that thelow binary frequency in the cluster is caused by the disruption of binaries due to dynamicalinteractions. There are two ways out of this dilemma: Either the initial binary frequency inthe Orion Nebula Cluster was lower than in Taurus-Auriga, or the Orion Nebula Cluster wasoriginally much denser and dynamically more active. A detailed report of this work has beenpublished in Astronomy Astrophysics (Kohler et al. 2006).
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