2012MNRAS.422.2560S

We present new MIDI interferometric and VISIR spectroscopic observations of HD 113766 and HD 172555. Additionally, we present VISIR 11-{img}m and 18-{img}m imaging observations of HD 113766. These sources represent the youngest (16 and 12 Myr old, respectively) debris disc hosts with emission on ≪ 10 au scales. We find that the disc of HD 113766 is partially resolved on baselines of 42-102 m, with variations in resolution with baseline length consistent with a Gaussian model for the disc with a full width at half-maximum (FWHM) of 1.2-1.6 au (9-12 mas). This is consistent with the VISIR observations which place an upper limit of 0.14 arcsec (17 au) on the emission, with no evidence for extended emission at larger distances. For HD 172555, the MIDI observations are consistent with complete resolution of the disc emission on all baselines of lengths 56-93 m, putting the dust at a distance of >1 au (>35 mas). When combined with limits from TReCS imaging, the dust at ∼10 {img}m is constrained to lie somewhere in the region of 1-8 au. Observations at ∼18 {img}m reveal extended disc emission which could originate from the outer edge of a broad disc, the inner parts of which are also detected but not resolved at 10 {img}m, or from a spatially distinct component. These observations provide the most accurate direct measurements of the location of the dust at 1-8 au that might originate from the collisions expected during terrestrial planet formation. These observations provide valuable constraints for models of the composition of discs at this epoch and provide a foundation for future studies to examine in more detail the morphology of debris discs.