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We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauristars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whosemedian ages lie in the <1 to 2 Myr range. The median mid-infrared spectra ofobjects in these three regions are similar in shape, suggesting, on average,similar disk structures. When normalized to the same stellar luminosity, themedians follow each other closely, implying comparable mid-infrared excessemission from the circumstellar disks. We use the spectral index between 13 and31 micron and the equivalent width of the 10 micron silicate emission featureto identify objects whose disk configuration departs from that of a continuous,optically thick accretion disk. Transitional disks, whose steep 13-31 micronspectral slope and near-IR flux deficit reveal inner disk clearing, occur withabout the same frequency of a few percent in all three regions. Objects withunusually large 10 micron equivalent widths are more common (20-30%); theycould reveal the presence of disk gaps filled with optically thin dust. Basedon their medians and fraction of evolved disks, T Tauri stars in Taurus andChamaeleon I are very alike. Disk evolution sets in early, since already theyoungest region, the Ophiuchus core (L1688), has more settled disks with largergrains. Our results indicate that protoplanetary disks show clear signs of dustevolution at an age of a few Myr, even as early as ~1 Myr, but age is not theonly factor determining the degree of evolution during the first few millionyears of a disk's lifetime.

Disk Evolution in the three Nearby Star-Forming Regions of Taurus, Chamaeleon, and Ophiuchus