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L. Hartmann (Harvard-Smithsonian Center for Astrophysics)
Studies of circumstellar disk emission from systems of ages 1 - 10 Myr are required to understand the evolution of disk gas and dust and its connection with planet formation. Large samples of stars are essential to explorations of disk evolution because there is a wide dispersion of disk properties among stars of approximately the same mass and age. Most investigations to date have focussed on disks among easily-found, very young stars (ages < 3 Myr), or debris disks around small samples of nearby, much older (ages > 20 Myr) stars, which leaves the expected epoch of (giant) planet formation at ~ 3-10 Myr relatively unexplored.
Observations of star clusters with appropriate ages can be used to follow the major coagulation phases in planet-forming disks. Such clusters are relatively rare, requiring study of objects at larger distances than well-probed by IRAS. SIRTF will provide the sensitivity needed to carry out such programs by detecting dusty disk emission in the wavelength range from 3 to 70 \mum. I will review our current understanding of young dust and gas disks, highlight some current problems of active interest, outline some general theoretical expectations for disk evolution, and summarize SIRTF's potential to make substantial contributions to our understanding of protoplanetary disks.