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R. Jayawardhana (CfA)
Being the nearest group of young stars, the TW Hydrae Association offers a unique laboratory to study disk evolution and planet formation. Furthermore, its estimated age of ~10 million years (Myr) provides a strong constraint on disk evolution timescales and fills a significant gap in the age sequence between ~1-Myr-old T Tauri stars in molecular clouds and ~50-Myr-old nearby open clusters. It has been suggested that circumstellar disks evolve from optically thick to optically thin structures in about 10 Myr. That transition may mark the assembly of grains into planetesimals, or clearing of the disk by planets. Indeed, circumstellar disks of the TW Hya Association members exhibit a wide variety behavior, from classical T Tauri accreting disks, to planetary debris systems, to systems without measureable disk emission at wavelengths shorter than 10 microns. The diverse disk properties suggest that the TW Hya stars are at an age when disks are rapidly evolving, through coagulation of dust and dissipation of gas.