Future solar systems

Widely regarded as pathological variable stars - with erratic photometric a nd spectroscopic behavior of unknown physical origin - 20 years ago, T Taur i stars (TTSs) turned out in the last decade to be promising laboratories f or observing the formation of solar systems such as ours. This is because c ircumstellar, presumably protoplanetary disks were found to surround a larg e fraction of them. While evidence for disks was primarily indirect until 1 995. recent high resolution imaging confirmed earlier claims that the infra red (IR) and ultraviolet (UV) excesses seen in the spectral energy distribu tion (SED) of these stars were due to disk emission. The activity displayed by young stellar objects at all wavelengths is due to the interaction betw een the circumstellar disk and the magnetized star and to non-stationary ac cretion/ejection phenomena. In this review, we briefly summarize properties of these young solar-type stars and describe their circumstellar disks in some detail. focusing on current optical. infrared and millimeter high angu lar resolution observations that now allow us to resolve the disks. (C) 200 1 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.