Dusty discs around evolved stars.

In this poster we report upon our programme to search, characterize and model circumstellar dusty discs around evolved stars.

It was realised some years ago that dusty discs around post-AGB stars are common. Famous examples, for which the disc is resolved, are the Red Rectangle (Men'shchikov et al.,2002,A&A 393,867) and AC Her (Jura et al.,2000,ApJ 541,264). But also in other objects, the SED and circumstellar kinematics point to a disc rather than to an expanding outflow. A SED characteristic seemed to be the presence of a near-IR excess and thus a broad range in dust-temperatures. The spectroscopy of those discs obtained with ISO showed that the dust features of the stable geometries are different: typically, a higher crystallisation fraction than in normal post-AGB outflows is observed (Molster et al.,1999,Nature 401,563) and the presence of large grains is needed to explain the mm and cm continuum measurements (Jura et al., 1997,ApJ 474,741). Moreover, mixed chemistry is often observed with typically cool O-rich features and hotter more recent C-rich material.

The variable class of RV Tauri stars are generally acknowledged to harbour a stable disc as well. The main indication was that many RV Tauri stars display both a near-IR excess (Evans,1985,MNRAS 217, 493) and a far IR-excess as found by IRAS (Jura,1986,ApJ 309,732).

The formation and evolution of such discs are not well understood, however. Binarity turns out to be a main ingredient in understanding these systems and is expected to be a widespread phenomenon in that class of objects. Therefore a selection of post-AGB and RV Tauri stars are subject to a radial velocity monitoring programme with the Euler Telescope.

We observed those objects with TIMMI2 to investigate the 10 μ m region. The selected objects, which are suspected binaries, have IR colours that are indicative of the presence of a dusty circumstellar disc rather than of a dusty outflow. By a systematic spectral study we want to gain insight into the circumstellar dust processing. More in particular, we want to focus on the study of the crystallisation process, which was discovered to be very efficient in long-lived dusty reservoirs but which is, as yet, badly understood.