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Abstract

Circumstellar discs serve as the progenitors of planetary systems. Their inner regions (from <1 au to a few astronomical units) are where terrestrial (i.e. Earth-like) planets form. Structures closely connected to planet formation, such as spirals, rings, or gaps, have been observed in the outer disc regions (∼20– 400 au). These outer regions are being intensively studied with instruments like SPHERE at near-infrared wavelengths or ALMA in the (sub-)millimetre. The innermost regions, on the other hand, have only been made accessible recently by interferometers with high spatial resolutions in the near- and mid-infrared (e.g. AMBER, MIDI, PIONIER, and GRAVITY on the VLTI). In this thesis, I use spectro-interferometric observations with the VLTI/MATISSE instrument, covering a broad wavelength range in the mid-infrared from the L/M (2.8–4.2 µm/4.5–5 µm) up to the N band (8– 13 µm), to investigate the structures present in the innermost regions of planet-forming discs. MATISSE provides spatial resolutions sufficient to resolve the innermost disc regions for sources in nearby star formation regions (∼100 pc). I apply data reduction and parametric modelling to give astrophysical interpretations of the interferometric observations. First, I explore the circumstellar discs of the HD 142527 binary system, where a close passing (periapsis∼5 au) stellar companion greatly perturbs the disc around the primary. Using a a disc model with an off-centre Gaussian asymmetric component, I find a time-variable structure around the primary at a separation of∼1– 1.2 au over the course of our observations (2021, 2022, and 2023) close to periapsis. The complex structure in the disc seems to be strongly influenced by the companion passage and warrants further observations to enable more detailed modelling. Subsequently, I perform a statistical analysis of∼60 sources from the MATISSE GTO YSO survey. I derive L- vs. N -band sizes from modelling and combine them with simple disc models. In doing so, I find that many discs in the survey sample show signs of gaps or truncations. Here, the sizes increase when the gaps in the disc become larger, up to the point of no significant N -band emission (i.e. truncated), which show smaller sizes than for continuous discs. The sample contains many strongly asymmetric sources from which disc substructures are deduced.