We have searched for visual binaries with projected separations in the range 200–3000 AU ( 0 \aas@@fstack { \prime \prime } 1–1 \aas@@fstack { \prime \prime } 5 ) among a sample of 96 stars in the massive young NGC 6611 cluster , 60 of them being subsequently identified as high probability cluster members of mainly OB spectral type .
This is the first visual binary survey among such a large and homogeneous sample of high-mass stars .
We find an uncorrected binary frequency of 18 \pm 6 % over the surveyed separation range .
Considering only binaries with mass ratios q \geq 0.1 , we find that OB stars in NGC 6611 host more companions than solar-type field stars .
We derive mass ratios for the detected binaries from their near-infrared flux ratios and conclude that about half of the detected binaries have q \lesssim 0.2 , which does not contradict the assumption that companion masses are randomly drawn from the initial mass function .
There is no evidence in our sample that wide-binary properties depend upon the mass of the primary star .
The high frequency of massive binaries in a cluster as rich as NGC 6611 and the lack of a strong mass dependence of their properties are difficult to reconcile with the scenario whereby massive stars form as the result of mergers of smaller stars .
The canonical protostellar accretion scenario together with cloud fragmentation , on the other hand , can naturally explain most of the observed binary properties , although the very high stellar density in the protocluster is likely to require significant modification to that picture as well .