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Disclosing the structure of disks surrounding Herbig AeBe stars is importantto expand our understanding of the formation and early evolution of stars andplanets. We aim at revealing the sub-AU disk structure around the 10 Myr oldHerbig Be star HD100546 and at investigating the origin of its near andmid-infrared excess. We used AMBER/VLTI observations to resolve the K-bandemission and to constrain the location and composition of the hot dust in theinnermost disk. Combining AMBER observations with photometric and MIDI/VLTImeasurements from the litterature, we revisit the disk geometry using a passivedisk model based on 3D radiative transfer. We propose a model that includes atenuous inner disk made of micron-sized dust grains, a gap, and a massiveoptically thick outer disk, that successfully reproduces the interferometricdata and the SED. We locate the bulk of the K-band emission at ~0.26 AU.Assuming that this emission originates from silicate, we show that micron-sizedgrains are required to enable the dust to survive at such a distance from thestar. As a consequence, more than 40% of the K-band flux is related toscattering, showing that direct thermal emission is not sufficient to explainthe near-infrared excess. In the massive outer disk, large grains in themid-plane are responsible for the mm emission while a surface layer of smallgrains allows the mid and far infrared excesses to be reproduced. Such verticalstructure may be an evidence for sedimentation. The observations are consistentwith a model that includes a gap until ~13 AU and a total dust mass of ~0.008lunar mass inside it. These values together with the derived scale height (~2.5AU) and temperature (~220 K) at the inner edge of the outer disk (r=13 AU), areconsistent with recent CO observations.

The complex structure of the disk around HD100546: the inner few astronomical units