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

Context. A region of roughly half of the solar system scale around the star HD 100546 is known to be largely cleared of gas and dust, in contrast to the outer disc that extends to about 400 AU. However, some material is observed in the immediate vicinity of the star, called the inner disc. Studying the structure of the inner and the outer disc is a first step to establishing the origin of the gap between them and possibly link it to the presence of planets. Aims. We answer the question of how the dust is distributed within and outside the gap, and constrain the disc geometry. Methods. To discern the inner from the outer disc, we used the VLTI interferometer instrument MIDI to observe the disc in the mid-infrared wavelength regime where disc emission dominates in the total flux. Our observations exploited the full potential of MIDI, with an effective combination of baselines of the VLTI 1.8 m and of 8.2 m telescopes. With baseline lengths of 40 m, our long baseline observations are sensitive to the inner few AU from the star, and we combined them with observations at shorter, 15 m baselines, to probe emission beyond the gap at up to 20 AU from the star. We modelled the mid-infrared emission using radial temperature profiles, informed by prior works on this well-studied disc. The model is composed of infinitesimal concentric annuli emitting as black bodies, and it has distinct inner and outer disc components. Results. Using this model to simulate our MIDI observations, we derived an upper limit of 0.7 AU for the radial size of the inner disc, from our longest baseline data. This small dusty disc is separated from the edge of the outer disc by a large, ≈10 AU wide gap. Our short baseline data place a bright ring of emission at 11 ± 1 AU. This is consistent with prior observations of the transition 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 ± 8 and PA = 145 ± 5. These values are close to known estimates of the rim and disc geometry and suggest co-planarity. Signatures of brightness asymmetry are seen in both short and long baseline data, unequivocally discernible from any atmospheric or instrumental effects. Conclusions. Mid-infrared brightness is seen to be distributed asymmetrically in the vicinity of the gap, as detected in both short and long baseline data. The origin of the asymmetry is consistent with the bright disc wall, which we find to be 1-2 AU wide. The gap is cleared of micron-sized dust, but we cannot rule out the presence of larger particles and/or perturbing bodies.