The circumbinary disc around the J - type C - star IRAS 18006 - 3213 ⋆

Context. In the generally accepted, but poorly documented model, silicate J-type C-stars are binary objects for which the silicate emission originates from a circumbinary or a circumcompanion disc. Aims. We aim at testing this hypothesis by a thorough spectral and spatial observational study of one object: IRAS 18006-3213. Methods. We obtained, analysed and modeled high spatial resolution interferometric VLTI/MIDI observations on multiple baselines ranging from 45 m to 100 m. Results. All observations resolved the object and show the very compact nature of the N-band emission (∼ 30 mas). In addition, the highest spatial resolution data show a significant differential phase jump around 8.3 µm. This demonstrates the asymmetric nature of the N-band emission. Moreover, the single telescope N-band spectrum shows the signature of highly processed silicate grains. These data are used to confirm the model on silicate J-type C-stars for IRAS 18006-3213. We show that the most favourable model of the dust geometry is a stable circumbinary disc around the system, seen under an intermediate inclination. Conclusions. The data presented on the silicate J-type C-star IRAS 18006-3213 provide evidence that the oxygen rich dust is trapped in a circumbinary disc. The formation of this disc is probably linked to the binary nature of the central star.