Binary stars in the Orion Nebula Cluster

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: koehler@strw.leidenuniv.nl
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, Devon, UK
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Laboratoire d'Astrophysique de Grenoble, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
⁶ Department of Physics & Astronomy, UCLA, Los Angeles, CA 90095-1562, USA
⁷ ZAH, Landessternwarte, Königstuhl, 69117 Heidelberg, Germany

Received: 21 November 2005
Accepted: 26 July 2006

Abstract

We report on a high-spatial-resolution survey for binary stars in the periphery of the Orion Nebula Cluster, at 5–15 arcmin (0.65–2 pc) from the cluster center. We observed 228 stars with adaptive optics systems, in order to find companions at separations of – (60–500 AU), and detected 13 new binaries. Combined with the results of Petr (1998), we have a sample of 275 objects, about half of which have masses from the literature and high probabilities to be cluster members. We used an improved method to derive the completeness limits of the observations, which takes into account the elongated point spread function of stars at relatively large distances from the adaptive optics guide star. The multiplicity of stars with masses is found to be significantly larger than that of low-mass stars. The companion star frequency of low-mass stars is comparable to that of main-sequence M-dwarfs, less than half that of solar-type main-sequence stars, and 3.5 to 5 times lower than in the Taurus-Auriga and Scorpius-Centaurus star-forming regions. We find the binary frequency of low-mass stars in the periphery of the cluster to be the same or only slightly higher than for stars in the cluster core (<3 arcmin from C Ori). This is in contrast to the prediction of the theory that the low binary frequency in the cluster is caused by the disruption of binaries due to dynamical interactions. There are two ways out of this dilemma: Either the initial binary frequency in the Orion Nebula Cluster was lower than in Taurus-Auriga, or the Orion Nebula Cluster was originally much denser and dynamically more active.