Vol. 578
In section 6. Interstellar and circumstellar matter

The evolved circumbinary disk of AC Herculis: a radiative transfer, interferometric, and mineralogical study

The authors model the structure of the circumstellar material around the post-AGB binary and RV Tauri pulsator AC Her, including the spatial distribution of the amorphous and the crystalline dust. To this end, they use high-quality mid-IR interferometric data obtained with the MIDI/VLTI instrument, and analyze the visibilities and differential phases in combination with the full SED, using the MCMax radiative transfer code. All data can be accounted for by a disk model with inclination and position angle i = 50 ± 8◦ and PA = 305 ± 10◦. The inner disk radius is about an order of magnitude larger than the dust sublimation radius. The best-fit dust grain size distribution shows that significant grain growth has occurred, with a significant amount of mm-sized grains that have settled to the midplane of the disk. A large total dust mass ≥ 10−3 M⊙ is needed to fit the sub-mm fluxes. By assuming αturb = 0.01, a good fit is obtained with a small grain size power law index of 3.25, combined with a small gas/dust ratio ≤10. The resulting gas mass is compatible with recent estimates employing direct gas diagnostics. The spatial distribution of the forsterite is different from the amorphous dust, as more warm forsterite is needed in the surface layers of the inner disk. The disk in the AC Her system appears to be in a very evolved state, but the authors find strong similarities with the protoplanetary disk HD100546.