A disc inside the bipolar planetary nebula M2-9

Aims. Bipolarity in proto-planetary and planetary nebulae is associated with events occurring in or around their cores. Past infrared observations have revealed the presence of dusty structures around the cores, many in the form of discs. Characterising those dusty discs provides invaluable constraints on the physical processes that govern the final mass expulsion of intermediate mass stars. We focus this study on the famous M2-9 bipolar nebula, where the moving lighthouse beam pattern indicates the presence of a wide binary. The compact and dense dusty core in the centre of the nebula can be studied by means of optical interferometry. Methods. M2-9 was observed with VLTI/MIDI at 39-47 m baselines with the UT2-UT3 and UT3-UT4 baseline configurations. These observations are interpreted using a dust radiative transfer Monte Carlo code. Results. A disc-like structure is detected perpendicular to the lobes, and a good fit is found with a stratified disc model composed of amorphous silicates. The disc is compact, 25 x 35 mas at 8 mu m and 37 x 46 mas at 13 mu m. For the adopted distance of 1.2 kpc, the inner rim of the disc is similar to 15 AU. The mass represents a few percent of the mass found in the lobes. The compactness of the disc puts strong constraints on the binary content of the system, given an estimated orbital period 90-120 yr. We derive masses of the binary components between 0.6-1.0 M-circle dot for a white dwarf and 0.6-1.4 M-circle dot for an evolved star. We present different scenarios on the geometric structure of the disc accounting for the interactions of the binary system, which includes an accretion disc as well.