Infrared long-baseline interferometry provides very high spatial
resolution at scales of milli-arcseconds and thus allows to directly
observe the innermost regions of the circumstellar environment of
massive stars, at linear dimensions of about 1 AU.
We will present the results of recent interferometric studies of
several massive young stellar objects at near- and mid-infrared
wavelengths. These data allow us to investigate the geometry of the
inner circumstellar dust- and gas-distribution and to disentangle
emission from warm dust, hot gas in accretion streams, and stellar
winds. In some objects, the interferometric data also reveal changes
of the dust grain properties with radial distance from the star,
providing direct evidence for dust processing that may be the first
step of the planet formation. These data provide strong constraints
for theoretical formation scenarios for massive stars and new insight
into the physical processes related to accretion, outflows, and winds.
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