Multiplicity of the massive stars in the Orion Nebula cluster

We present bispectrum speckle interferometry observations of 13 bright Orio n 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 wi th 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 ma sses 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 c ompanion mass function similar to the field IMF. Considering both, the visu al and the spectroscopic companions of the 13 target stars, the total numbe r of companions is at least 14. Extrapolation with correction for the unres olved systems suggests that there are at least 1.5 companions per primary s tar on average. This number is clearly higher than the mean number of simil ar to 0.5 companions per primary star found for the low-mass stars in the O rion 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 sys tems in the dense Orion Nebula cluster than for low-mass stars. (C) 1999 El sevier Science B.V. All rights reserved.