Multiplicity of the massive stars in the Orion Nebula cluster

We present bispectrum speckle interferometry observations of 13 bright Orion Nebula cluster member stars of spectral type O or B. Diffraction-limited images with a resolution lambda/D of 75 mas in the K'-band were obtained with the SAO 6 m telescope. In our speckle images we find 8 visual companions in total. Using the flux ratios of the resolved systems to estimate the masses of the companions, we find that the systems generally have mass ratios below 1/2. The distribution of mass ratios seems to be consistent with a companion mass function similar to the field IMF. Considering both, the visual and the spectroscopic companions of the 13 target stars, the total number of companions is at least 14. Extrapolation with correction for the unresolved systems suggests that there are at least 1.5 companions per primary star on average. This number is clearly higher than the mean number of similar to 0.5 companions per primary star found for the low-mass stars in the Orion Nebula cluster as well as in the field population. This suggests that a different mechanism is at work in the formation of high-mass multiple systems in the dense Orion Nebula cluster than for low-mass stars. (C) 1999 Elsevier Science B.V. All rights reserved.