High-resolution ALMA observations of compact discs in the wide-binary system Sz 65 and Sz 66^★

¹ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
e-mail: jmiley73@gmail.com
² National Astronomical Observatory of Japan, NAOJ Chile Observatory, Los Abedules 3085, Oficina 701, Vitacura, Santiago, Chile
³ School of Physics & Astronomy, University of Leeds, Sir William Henry Bragg Building, Woodhouse Lane, Leeds LS2 9JT, UK
⁴ Department of Astronomy and Astrophysics, Penn State University, 525 Davey Laboratory, University Park, PA 16802, USA
⁵ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁶ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁷ Center for Astrophysics | Harvard &Smithsonian, Cambridge, MA 02138, USA
⁸ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, 06304 Nice, France
⁹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁰ Department of Astronomy, Columbia University, 538 W. 120th Street, Pupin Hall, New York, NY 10027, USA
¹¹ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
¹² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹³ Department of Physics and Astronomy, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330, USA
¹⁴ Department of Physics and Astronomy, 6100 Main MS-550, Rice University, Houston, TX 77005, USA

Received: 8 June 2023
Accepted: 9 November 2023

Abstract

Context. Substructures in disc density are ubiquitous in the bright extended discs that are observed with high resolution. These substructures are intimately linked to the physical mechanisms driving planet formation and disc evolution. Surveys of star-forming regions find that most discs are in fact compact, less luminous, and do not exhibit these same substructures. It remains unclear whether compact discs also have similar substructures or if they are featureless. This suggests that different planet formation and disc evolution mechanisms operate in these discs.

Aims. We investigated evidence of substructure within two compact discs around the stars Sz 65 and Sz 66 using high angular resolution observations with ALMA at 1.3 mm. The two stars form a wide-binary system with 6″.36 separation. The continuum observations achieve a synthesised beam size of 0″.026 × 0″.018, equivalent to about 4.0 × 2.8 au, enabling a search for substructure on these spatial scales and a characterisation of the gas and dust disc sizes with high precision.

Methods. 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 ¹²CO (2–1) emission line. Comparisons were made with highresolution observations of compact discs and radially extended discs.

Results. We find evidence of substructure in the dust distribution of Sz 65, namely a shallow gap centred at ≈20 au, with an emission ring exterior to it at the outer edge of the disc. Ninety percent of the measured continuum flux is found within 27 au, and the distance for ¹²CO is 161 au. 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 64 au.

Conclusions. 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.