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Spectroscopic multiplicity surveys of O stars in young clusters and OBassociations have revealed that a large portion ($\sim$ 70%) of these massivestars (M$_{i}$ $\gt$ 15 $M_{\odot}$) belong to close and short-period binaries(physical separation d $\lt$few au). Despite the recent and significantprogress, the formation mechanisms leading to such close massive multiplesystems remain to be elucidated. As a result, young massive close binaries (orhigher-order multiple systems) are unique laboratories to figure out thepairing mechanism of high-mass stars. We present the first VLTI/GRAVITYobservations of six young O stars in the M17 star-forming region ($\lesssim$ 1Myr) and two additional foreground stars. From the interferometric modelfitting of visibility amplitudes and closure phases, we search for companionsand measure their positions and flux ratios. Combining the resulting magnitudedifference with atmosphere models and evolutionary tracks, we further constrainthe masses of the individual components. All of the six high-mass stars are inmultiple systems, leading to a multiplicity fraction (MF) of 100%, yielding a68% confidence interval of 94-100%. We detect a total number of 9 companionswith separations up to 120 au. Including previously identified spectroscopiccompanions, the companion fraction of the young O stars in our sample reaches2.3$\pm$0.6. The derived masses span a wide range from 2.5 to 50 $M_{\odot}$,with a great tendency towards high-mass companions. While based on a modestsample, our results clearly indicate that the origin of the high degree ofmultiplicity is rooted in their star formation mechanism. No clear evidence forone of the competing concepts of massive star formation (core accretion orcompetitive accretion) could be found. However, our results are compatible withmigration as a scenario for the formation of close massive binaries.

On the origin of close massive binaries in the M17 star-forming region