Sz 65 and Sz 66 ALMA observations

The two stars form a wide-binary system with 6.36" separation. The continuum observations achieve a synthesised beam size of 0.026"x0.018", equivalent to about 4.0x2.8au, enabling a search for substructure on these spatial scales and a characterisation of the gas and dust disc sizes with high precision. We analysed the data in the image plane through an analysis of reconstructed images, as well as in the uv plane by non-parametrically modelling the visibilities and by an analysis of the ^12^CO (2-1) emission line.

Comparisons were made with high-resolution observations of compact discs and radially extended discs. We find evidence of substructure in the dust distribution of Sz 65, namely a shallow gap centred at ~20au, with an emission ring exterior to it at the outer edge of the disc. Ninety percent of the measured continuum flux is found within 27au, and the distance for ^12^CO is 161au.

The observations show that Sz 66 is very compact: 90% of the flux is contained within 16 au, and 90% of the molecular gas flux lies within 64au. While the overall prevalence and diversity of substructure in compact discs relative to larger discs is yet to be determined, we find evidence that substructures can exist in compact discs