Dust-grain processing in circumbinary discs around evolved binaries. The RV Tauri spectral twins RU Centauri and AC Herculis

Context. We study the structure and evolution of the circumstellar discs around evolved binaries and their impact on the evolution of the central system. Aims. By combining a wide range of observational data and techniques, we aim to study in detail the binary nature of RU Cen and AC Her, as well as the structure and mineralogy of the circumstellar environment. Methods. We combine a multi-wavelength observational program with a detailed 2D radiative transfer study. Our radial velocity program is instrumental in the study of the nature of the central stars, while our Spitzer spectra complimented with the broad-band spectral energy distribution (SED) are used to constrain mineralogy, grain sizes and physical structure of the circumstellar environment. Results. We determine the orbital elements of RU Cen showing that the orbit is highly eccentric with a large velocity amplitude despite the rather long period of 1500 days. The infrared spectra of both objects are very similar and the spectral dust features are dominated by magnesium-rich crystalline silicates. The small peak-to-continuum ratios are interpreted as being due to large grains. Our model contains two components with a cold midplain dominated by large grains, and the near- and mid-IR which is dominated by the emission of smaller silicates. The infrared excess is well modelled assuming a hydrostatic passive irradiated disc. The profile-fitting of the dust resonances shows that the grains must be very irregular. Conclusions. These two prototypical RV Tauri pulsators with circumstellar dust are binaries where the dust is trapped in a stable disc. The mineralogy and grain sizes show that the dust is highly processed, both in crystallinity and grain size. The cool crystals show that either radial mixing is very efficient and/or that the thermal history at grain formation has been very different from that in outflows. The physical processes governing the structure of these discs are very similar to those observed in protoplanetary discs around young stellar objects.