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Description
Circumstellar discs are essential for high mass star formation, while multiplicity, in particular binarity, appears to be an inevitable outcome since the vast majority of massive stars (>8M_{sun}_) are found in binaries (up to 100%). We spatially resolve and constrain the sizes of the dust and ionised gas emission of the innermost regions towards a sample of MYSOs for the first time, and provide high-mass binary statistics of young stars at 2-300au scales. We observe six MYSOs with VLTI (GRAVITY, AMBER), to resolve and characterise the 2.2um hot dust emission originating from the inner rim of circumstellar discs around MYSOs, and the associated Br{gamma} emission from ionised gas. We fit simple geometrical models to the interferometric observables, and determine the inner radius of the dust emission. We place MYSOs with K-band measurements in a size-luminosity diagram for the first time, and compare our findings to T Tauris and Herbig AeBes. We also compare the observed K-band sizes to the sublimation radius predicted by three different disc scenarios. Lastly, we apply binary geometries to trace close binarity among MYSOs. When the inner sizes of MYSOs are compared to those of lower mass Herbig AeBe and T Tauri stars, they appear to follow a universal trend at which the sizes scale with the square-root of the stellar luminosity. The Br{gamma} emission originates from a similar or somewhat smaller and co-planar area compared to the 2.2um continuum emission. We discuss this new finding with respect to disc-wind or jet origin. Finally, we report an MYSO binary fraction of 17-25% at milli-arcsecond separations (2-300au). The size-luminosity diagram indicates that the inner regions of discs around young stars scale with luminosity independently of the stellar mass. At the targeted scales (2-300au), the MYSO binary fraction is lower than what was previously reported for the more evolved main sequence massive stars, which, if further confirmed, could implicate the predictions from massive binary formation theories. Lastly, we spatially resolve the crucial star/disc interface in a sample of MYSOs, showing that au-scale discs are prominent in high-mass star formation and similar to their low-mass equivalents.
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