The first interferometric survey in the K-band of massive YSOs. On the
  hot dust, ionised gas, and binarity at au scales
  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 (> 8 Msun) are found in binaries (up to
100%). We constrain the sizes of the dust and ionised gas (Brgamma) emission of
the innermost regions towards a sample of six MYSOs, and provide high-mass
binary statistics of young stars at 2-300 au scales using VLTI (GRAVITY, AMBER)
observations. We determine the inner radius of the dust emission and 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. The inner sizes of MYSOs, Herbig AeBe and T Tauri stars appear to
follow a universal trend at which the sizes scale with the square-root of the
stellar luminosity. The Brgamma emission originates from somewhat smaller and
co-planar area compared to the 2.2 {\mu}m 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 (2-300 au). 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 (2-300 au), 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.
