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A region of roughly half of the solar system scale around the star HD 100546is largely cleared of gas and dust, in contrast to the bright outer disc.However, some material is observed in the immediate vicinity of the star. Weinvestigate how the dust is distributed within and outside the gap, andconstrain the disc geometry with mid-infrared interferometric observationsusing VLTI/MIDI. With baseline lengths of 40m, our long baseline observationsare sensitive to the inner few AU from the star, and we combined them withobservations at shorter, 15m baselines, to probe emission beyond the gap at upto 20AU from the star. We modelled the mid-infrared emission using radialtemperature profiles. Our model is composed of infinitesimal concentric annuliemitting as black bodies, and it has distinct inner and outer disc components.We derived an upper limit of 0.7AU for the radial size of the inner disc, fromour longest baseline data. This small dusty disc is separated from the edge ofthe outer disc by a large, roughly 10AU wide gap. Our short baseline data placea bright ring of emission at 11+-1AU, consistent with prior observations of thetransition region between the gap and the outer disc, known as the disc wall.The inclination and position angle are constrained by our data to i=53+-8degand PA=145+-5deg. Compared to the rim and outer disc geometry this suggestsco-planarity. Brightness asymmetry is evident in both short and long baselinedata, and it is unequivocally discernible from any atmospheric or instrumentaleffects. The origin of the asymmetry is consistent with the bright disc wall,which we find to be 1-2AU wide. The gap is cleared of micron-sized dust, but wecannot rule out the presence of larger particles and/or perturbing bodies.

Resolving HD 100546 disc in the mid-infrared: Small inner disc and asymmetry near the gap