The multiplicity of newborn high-mass stars

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The origin of massive stars (with masses over 10 solar masses) is one of the main unsolved problems in star formation and is currently the subject of heated debate. While the initial mass function of massive stars has long been recognised as an important topic, the binary properties of massive stars have been much less studied. However, it is becoming increasingly evident that only few massive stars appear to be single, and they may even have been part of binary systems that were dynamically dissolved. Understanding massive star formation is therefore equivalent to understanding massive binary formation. The companion star fraction for massive stars in the Orion Trapezium is 1.5, whereas it is 0.5 for low-mass stars in Orion. This discrepancy is a strong indicator that the formation of high-mass and low-mass binaries may be different, and that massive star formation is not likely to be merely a scaled-up version of lower-mass star formation. No less than four different hypotheses have been forwarded to explain the birth of massive binaries: (1) Fragmentation of massive disks around massive stars, (2) Bondi-Hoyle accretion onto a low-mass protobinary, (3) Encounters and tidal capture of intermediatemass protostars in a dense cluster, and (4) Dynamical three-body capture in a dense cluster. In order to test these theories, we need a more complete understanding of the binarity of massive stars at very young ages, when dynamical interactions have not yet been fully played out. We propose to embark on a systematic study of the approximately 80 massive stars in the Orion OB1 association, search for binaries, and determine their characteristics. This material can then be compared by theorists with their predictions of the four hypotheses for massive binary formation listed above. In addition we note that only preliminary atmospheric modelling has been made on a few of these OB stars and that our spectroscopic material will be ideally suited for such an analysis.