PROBING MASS LOSS VARIATIONS OF AGB STARS WITH DETACHED SHELLS DIFFERENCES IN MASS LOSS BEHAVIOR FOR C- AND O-RICH AGB STARS

on the upper agb, stellar evolution is determined by mass loss. it is now well established from observations that mass loss is subject to large temporal variations, or interruptions (willems and de jong 1988, a&a 196, 173; zijlstra et al. 1992, a&a 265, l5), probably related to the luminosity changes that occur in thermally pulsating agb stars. as a natural consequence of such mass loss variability, detached shells are expected to develop repeatedly. the existence of detached shells around cstars is not only indicated by their spectral energy distributions but has been confirmed by spatially resolved co maps (olofsson et al. 1996, a&a 311, 587) and some highresolution iras images (waters et al. 1994, a&a 281, l1). however, comparable observations of detached shells around orich agb stars have not been obtained up to now, suggesting that this phenomenon is less frequent or less pronounced for these objects. if confirmed, such a situation would imply significant differences in the mass loss behavior between crich and orich agb stars. the present proposal is dedicated to address this question by searching for detached dust shells around orich objects and analyzing their properties in comparison to those of crich stars. guided by our hydrodynamical models which show that the existence of a cold detached dust shell around orich sources is correlated with weak or almost nonexisting silicate features (see enclosed fig. 1), we propose a sample of five promising candidates (3 of them known to be extended) for observation by iso. we supplemented our sample by 3 new cstar candidates (also known to be extended) showing significant 60 and 100 micron (um) excess fluxes after correction for extension. data analysis will make use of elaborate timedependent radiation hydrodynamics models of models of agb winds which shall serve as an important new tool for a physical interpretation of the i truncated! [truncated!, Please see actual data for full text]