DISK EVOLUTION IN THE THREE NEARBY STAR-FORMING REGIONS OF TAURUS, CHAMAELEON, AND OPHIUCHUS

Abstract

We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauri\nstars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whose\nmedian ages lie in the <1 to 2 Myr range. The median mid-infrared spectra of\nobjects in these three regions are similar in shape, suggesting, on average,\nsimilar disk structures. When normalized to the same stellar luminosity, the\nmedians follow each other closely, implying comparable mid-infrared excess\nemission from the circumstellar disks. We use the spectral index between 13 and\n31 micron and the equivalent width of the 10 micron silicate emission feature\nto identify objects whose disk configuration departs from that of a continuous,\noptically thick accretion disk. Transitional disks, whose steep 13-31 micron\nspectral slope and near-IR flux deficit reveal inner disk clearing, occur with\nabout the same frequency of a few percent in all three regions. Objects with\nunusually large 10 micron equivalent widths are more common (20-30%); they\ncould reveal the presence of disk gaps filled with optically thin dust. Based\non their medians and fraction of evolved disks, T Tauri stars in Taurus and\nChamaeleon I are very alike. Disk evolution sets in early, since already the\nyoungest region, the Ophiuchus core (L1688), has more settled disks with larger\ngrains. Our results indicate that protoplanetary disks show clear signs of dust\nevolution at an age of a few Myr, even as early as ~1 Myr, but age is not the\nonly factor determining the degree of evolution during the first few million\nyears of a disk's lifetime.\n