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The Morphologies and Lifetimes of Transitional Protoplanetary Disks

I describe new constraints on the lifetimes and morphologies of transitional protoplanetary disks from observations of 1--10 Myr old stars with the \textit{Spitzer Space Telescope}. New Spitzer results clearly show evidence for two kinds of transitional disks and thus two main disk evolutionary pathways: disks which form an inner hole/gap and clear from the inside out and disks that deplete more homologously. Analyzing the disk populations of 1--10 Myr old clusters such as Taurus, IC 348, NGC 2362, and $η$ Cha show that the mean transitional disk lifetime must be an appreciable fraction of the mean protoplanetary disk lifetime: $\approx$ 1 Myr out of 3--5 Myr. The varieties of transitional disk SEDs and correlations with other disk diagnostics are consistent with multiple mechanisms responsible for clearing disks.