Transition disc nature of post-AGB binary systems confirmed by mid-infrared interferometry

TL;DR

This study uses mid-infrared interferometry to confirm that circumbinary discs around post-AGB binaries are transition discs with large dust-free inner cavities, similar to protoplanetary discs but larger in size.

Contribution

It provides direct observational evidence of transition disc structures around post-AGB binaries using high-angular resolution mid-IR data.

Findings

Inner dust rims are 2.5 to 7.5 times larger than dust sublimation radii.

Discs without near-IR excess have rims similar to theoretical sublimation radii.

Inner rim sizes are 3-25 au, larger than those in young star transition discs.

Abstract

Many properties of circumbinary discs around evolved post-asymptotic giant branch (post-AGB) binary systems are similar to those of protoplanetary discs around young stars. The deficits of near-infrared (near-IR) flux in the spectral energy distributions (SEDs) of these systems hints towards large dust-free cavities that are reminiscent of transition discs as are commonly observed around young stars. We aim to assess the size of the inner rim of 6 post-AGB binary systems with lack in the near-IR like this. We used resolved mid-infrared (mid-IR) high-angular resolution observations of VLTI/MATISSE and VLTI/MIDI. We compared these inner rim sizes to 5 systems with available MATISSE data that were identified to host a disc starting at the dust sublimation radius. We used geometric ring models to estimate the inner rim sizes, the relative flux contributions of the star, the ring, and an over-resolved emission, the orientation of the ring, and the spectral dependences of the components. We find that the inner dust rims of the targets with a lack of near-IR excess in their SEDs are 2.5 to 7.5 times larger than the theoretical dust sublimation radii, and inner rim sizes of the systems that do not show this deficit are similar to those of their theoretical dust sublimation radii. The physical radii of the inner rims of these transition discs around post-AGB binaries are 3-25 au, which are larger than the disc sizes inferred for transition discs around young stars with VLTI/MIDI. With mid-IR interferometric data, we directly confirm the transition disc nature of six circumbinary discs around post-AGB binary systems. Future observational and modelling efforts are needed to progress in our understanding of the structure, origin, and evolution of these transition discs