2021A&A...650L..13C

Context. The presence of stable, compact circumbinary discs of gas and dust around post-asymptotic giant branch (post-AGB) binary systems has been well established. We focus on one such system, IRAS 08544-4431.
Aims. We present an interferometric multi-wavelength analysis of the circumstellar environment of IRAS 08544-4431. The aim is to constrain different contributions to the total flux in the H-, K-, L-, and N-bands in the radial direction.
Methods. The data obtained with the three current instruments on the Very Large Telescope Interferometer (VLTI), VLTI/PIONIER, VLTI/GRAVITY, and VLTI/MATISSE, range from the near-infrared, where the post-AGB star dominates, to the mid-infrared, where the disc dominates. We fitted the following two geometric models to the visibility data to reproduce the circumbinary disc: a ring with a Gaussian width and a flat disc model with a temperature gradient. The flux contributions from the disc, the primary star (modelled as a point source), and an over-resolved component were recovered along with the radial size of the emission, the temperature of the disc as a function of radius, and the spectral dependencies of the different components.
Results. The trends of all visibility data were well reproduced with the geometric models. The near-infrared data were best fitted with a Gaussian ring model, while the mid-infrared data favoured a temperature gradient model. This implies that a vertical structure is present at the disc inner rim, which we attribute to a rounded puffed-up inner rim. The N-to-K size ratio is 2.8, referring to a continuous flat source, analogues to young stellar objects.
Conclusions. By combining optical interferometric instruments operating at different wavelengths, we can resolve the complex structure of circumstellar discs and study the wavelength-dependent opacity profile. A detailed radial, vertical, and azimuthal structural analysis awaits a radiative transfer treatment in 3D to capture all non-radial complexity.