The evolution of proto-planetary discs

scientific abstract the discovery of cool material around mainsequence stars such as vega and beta pictoris has maybe been most the impacting new result from the iras mission, and is widely seen as one of the most promising new facts that may lead to a better understanding of the formation of planetary systems. the spectroscopic capabilities of iso provide the ideal tool for studying the circumstellar emission in vegatype stars in more detail. measurements from the ground have provided a first hint that cometarylike silicate emission is present around beta pic, hd 98800, and 51 oph. with sws, this feature can be studied much more accurately and can be detected in other stars as well. the precise determination of the onset of the ir excess provides an important test on the size of the circumstellar grains. important emphasis is put on the study of the evolution of the protoplanetary disks from ysos (studied in the wesselius et al. proposal) to mainsequence stars. we have chosen to focus on objects in the 2solarmass range (herbig ae/be stars), because it appears that for such objects the transition toward the main sequence can be studied in objects that are relatively free from background emission. the energy distributions of these objects suggest that the ir excess first weakens in the midir region, i.e. the spectral region covered by sws. an interesting possibility is that this happens through the formation of larger bodies. observation summary. measurements with sws will be performed for all stars for which the expected s/n ratio is sufficient. the goal of these observations is to detect and describe solidstate features and to determine accurately the onset of the ir excess. only mediumresolution (r 200) aot01 observations (full scans) are requested. for all objects a phots (aot40) 612 micron scan will be observed, in order to describe accurately the photospheric flux, and so to determi truncated!, Pl [truncated!, Please see actual data for full text]